There are 2 reasons why it is generally advisable to use each receiving address only once:
1. Your privacy is protected a little more, as it is not directly visible on the blockchain that these two payments went to the same wallet/person (but you should be aware that when you use such funds in a transaction together, the relationship becomes apparent)
2. There is a remote possibility that the security of a private key is a little bit weaker after you have spent the funds sent to the corresponding address, because for that transaction the public key needs to be presented (as long as you don't spend funds that were sent to an address only the address is known, which is derived from the public key but not the key itself.) I don't know whether this is an actual risk or only a hypothetical one, but especially when you deal with larger amounts of bitcoins it is better to avoid risks.

Onkel Paul

If that actually were your private key, your coins would be gone faster than you can say "wallet". Apparently it is not a private key belonging to a funded bitcoin address, though.

Onkel Paul

When it's up to $1000 again you will be glad that you lost your wallet password (if you happen to remember it then.)

Onkel Paul

Nice catch!
But I think Satoshi would probably refuse to keep e-mail contact with people using the nosy outlook.com e-mail system...

Onkel Paul

The bitcoins you bought at bitcoin.de are still in bitcoin.de's wallet. Your account shows a positive balance because you essentially bought the right to withdraw that amount. This is comparable to having some amount of money in a bank account. You can't spend it at any place where cash is needed but you can withdraw cash at an ATM.

To get the bitcoins into your wallet, you need to transfer them to a receiving address generated by your wallet. I don't know how bitcioin.de accounts work, but I suppose you can send bitcoins from there to any bitcoin address. So first you create a receiving address, then you create a transaction at bitcoin.de to send bitcoins from your account to your wallet.

In most cases, this transaction will be processed within a reasonably short time. Your wallet will probably see it within a few seconds as an unconfirmed transaction. Depending on your wallet, you might not be able to spend the bitcoins from that transaction before it has a number of confirmations.

You might also be able to send from bitcoin.de directly to some bitcoin recipient. However, since this transaction does not come from a wallet under your control, you might be unable to prove that you initiated the payment (in some situations, it is desirable to sign a message with the address from which bitcoins were sent, to prove that you are the owner of that address.)

Is this explanation clear enough?
If you speak german, you might also ask in the german subforum, there are a number of knowledgeable and helpful people there.

Onkel Paul

Firewall issues maybe?
I don't use Macs, so I don't know how likely it is that there are default rules that might prevent proper blockchain sync.
Have you had a look at the debug console of Bitcoin-QT? Does it provide any sort of meaningful info?

Onkel Paul

Well I was active in the forum before signature campaigns, and didn't join one for quite some time.
And even now that I have signature I don't really post much more than before (look at my post history). I try to post only when I can contribute some value to a thread.
The signature payment is nice but not high enough to make me write meaningless posts.

Onkel Paul

Whatever you will try, backup your wallet.dat first!

Your client is pretty old, upgrading the client to a current version is strongly recommended (consider it mandatory). That should fix the "you may need to upgrade or other nodes may need to upgrade" warning and allow the client to catch up with the blockchain relatively quickly.
After upgrading the client and syncing it your balance should be visible again. If it does not, you should stop the client, copy the wallet.dat from your backup to the right place, and restart the client.

Onkel Paul

As others have said, downloading and syncing the whole blockchain takes quite some time, so when you want to run a full node with a complete copy of the blockchain you just have to accept it.
However, SPV wallets are a very reasonable alternative - your bitcoins are just as safe with them as with bitcoin core (your private keys stay on your computer) but you don't need the huge overhead.
See https://en.bitcoin.it/wiki/Thin_Client_Security for some more info on technical details.

Onkel Paul

No, QR code is defined such that you can encode your information differently, but decoding will always result in the same information that you encoded (and the decoder will know how you encoded stuff, because that's part of the encoding).
For example, you may specify different amount of error correction info, so the total number of bits (small squares) in the QR code is different, but the actual encoded info is the same.
This is especially useful when you intend the QR code to be printed out on paper that may become dirty or slightly damaged. For display on a computer screen, you don't need to add much error correction.
QR is a pretty slick invention :-)

Onkel Paul

Actually, price determination is a pretty complex process.
If there is a market (enough buy and sell orders) then the price will be determined by supply and demand.
Between exchanges (and other places where buying or selling takes place) there is some amount of arbitrage (when you can buy BTC at one exchange for $285 and can sell them at another exchange for $295, some folks will buy on one exchange and sell on the other, adjusting supply, demand and prices on both exchanges.)

Without markets, determining a price is difficult. Remember the classic bitcoin pizza? At that time, a future price of $1 per Bitcoin was considered pretty much impossible, so Bitcoin was valued much less than that. Later on, when it became clear that you could not only buy pizza with BTC, people were gradually willing to value it more.

Onkel Paul

You can probably fool newbies just as you could fool them trying to sell gold-painted bricks as gold bars - you need to set up quite a bit of smoke and mirrors to hide the fact from them that you're not actually selling bitcoins.
Of course, people fall for the most obvious scams all the time (think of the "orphaned account" or bride scam stories) so there are probably people dumb enough to fall for this, too.

Onkel Paul

People can buy and sell at whatever rate they think is appropriate.
Localbitcoins does not create a true market where arbitrary buyers and sellers come together and define a price by combining their offers. So I would not trust these localbitcoins numbers, they are either made up (by parties collaborating to make the exchange rate look higher than it actually is) or happen due to extreme cases (for example, really small amounts where the buyer does not care if he pays a few dollars more than market rate as soon as he can get BTC quickly and anonymously.)

Onkel Paul

I haven't checked (and don't have time for it right now) but is the backend code somehow open source?
Would it be possible for someone who has a full node to run a mycelium backend for himself (and maybe some friends?)
I suppose there are trust issues, so connecting Mycelium to any random backend server might be not advisable, but for personal use it should be ok, and maybe some other trusted organizations would be willing to run Mycelium backends as well.

Onkel Paul

Looks like it is indeed slow.
After a while, I got some info displayed - looks like this address has received quite a number of dust transactions and someone is trying to consolidate them.
Did the guy who wants to send you money prove that he is the owner of this address (by signing using the corresponding private key?)
Apparently, the balance of that address changes quickly, so anything you see there (if you're lucky enough to see something) might not mean a lot some time later. The number I've seen on blockchain.info was something like the 0.0553btc mentioned in previous post, on coinplorer.com it shows 0.2485. Might be due to a number of unconfirmed transactions in the mempool, I suppose coinplorer.com ignores more of them than blockchain.info does.

Onkel Paul

The blockchain is up - last transaction just 5 minutes ago, lots of nodes and miners working, everything is just normal.

You probably mean blockchain.info. That site doesn't look down either. I don't have a blockchain.info wallet, so I don't know whether their wallet service works. You should use a wallet on your computer anyway...

Have you tried talking to their support? That might be more helpful than venting your frustration here in the forum.

Onkel Paul

The exchange rate between USD and BTC is pretty independent from amount and distribution of BTC. It is what some people are willing to pay, and other people are willing to accept. Demand varies with perceived usefulness and speculative future value, supply of old coins somewhat depends on previous price levels and mining costs, while supply of fresh coins is pretty much fixed by the block rewards. Miners will try to sell fresh coins above cost if possible, some might withhold their mined bitcoins if they feel the price is too low, but they can't withhold indefinitely since they have bills to pay.

All of these factors are not part of your computation, so I'd suggest that whatever values you use for your unknowns, the results will be meaningless.

Apart from that, I still think BTC is undervalued, and a serious increase in exchange rate is quite realistic. We need more actual use and easier accessibility for this to happen, though. Just my opinion.

Onkel Paul

Ohne deine Finanzsituation so ganz genau zu kennen, schätze ich doch, dass dein privater Bank Run das Finanzsystem wohl nicht zum Brennen bringen wird. Da muss schon ein bisschen mehr passieren :-)

Wir sind von den Eigenschaften der Cryptowährungen fasziniert und sehen sie als disruptiv an, aber für den globalen Finanzmarkt ist das alles noch Spielzeug. Die finden zwar die Blockchain-Technik interessant, aber beim Bitcoin selbst sind ihnen die Besitzverhältnisse viel zu suspekt, als dass sie das als ernsthafte Komponente in einem zukünftigen globalen System in Betracht ziehen. Das geht doch gar nicht an, dass da ein paar Nerds, die 2009 ihren Rechner mit sinnlosen Rechenoperationen beschäftigt haben, jetzt auf einmal über Millionenwerte verfügen!

Onkel Paul

They are paying but payments are seriously backlogged due to the transaction spam attack. The transaction containing my payment from last week took a couple of days to get included in a block.
Don't know whether that's the reason for your missing payments, though.

Onkel Paul

Im Bitcoin-Wiki ist das technisch detailliert erklärt: https://de.bitcoin.it/wiki/Transaktionsgeb%C3%BChren
Wie aktuell das ist, weiß ich nicht, habe mich länger nicht damit beschäftigt.
Als Fausregel gilt: 0.0001 BTC pro Kilobyte der Transaktion. Die Größe der Transaktion hängt vor allem von der Anzahl Inputs ab.
Normalerweise wirst du selten Transaktionen mit mehr als 2 Outputs versenden (1 Output für das Ziel, 1 Output für den Restbetrag, der wieder in deiner Wallet landet), das ist eher für Firmen interessant, die mehrere Zahlungen mit einer Transaktion erledigen wollen.
Die meisten Wallets berechnen eine passende Transaktionsgebühr selbst, du solltest allerdings darauf achten, eine aktuelle Wallet-Software zu haben, weil die Entwickler unter Umständen die Berechnung oder die Basis-Werte etwas anpassen.

Onkel Paul