Thank you very much! I would like to buy for example $ 200,000 of Bitcoin in CASH!
Appears that you have several problems to solve then.
There won't be a single direct seller or even sets of them with the capacity to accept a couple hundred grand in cash in exchange for bitcoins.
So your first problem is essentially comes down to ways to move cash (an offline non-reversible medium of exchange) into an electronic form that is compatible with bitcoin.
Of course, the banking system does this very well for not a whole lot of money. You simply deposit cash and send a wire transfer to an exchange that can accommodate the size of your order
Using the buy calculator from Clark Moody shows that you can accomplish a $200K USD purchase in a single buy and pay on average about 5% above spot (using the market depth at the time) at Mt. Gox.
Buying chunks at a time will give those who perform arbitrage against the other markets (e.g., the BTC/USD at BitFloor, the BTC/EUR at Intersango, etc) a chance to bring supply to you but at the same time doing that runs the risk of tipping off other traders that will front-run you by anticipating that you will be buying further.
But that's definitely something that can be accommodated solely on a single exchange without too much of a penalty.
Now that the value of all bitcoin's calculated at the market exchange rate (which some refer to as the "market cap") is about $100 milliion, a $200K purchase doesn't move the exchange rate like it used to. It is still a very significant amount though.
Knowing in advance the volume and the likely effect on price you then the derivatives available might be employed.
If you don't immediately need the entire bitcoin proceeds from your purchase at the exchange, then you might buy CALL options for some of your purchase. This allows you to have a known upper bound on price for those. So then you start buying bitcoins on the open market for the remaining quantity you don't already have CALL options for. If the exchange rate remains below your exercise price you can keep buying and sell to close your CALL option to get back most or all of the premium you paid for it.
There are quote a few methods available to you to accomplish your aim.
Now, I was purposely being obtuse here. You specifically mentioned cash, and you probably did not want to consider the option of cash being sent through the bankiing system.
So that brings to the fore the other problems you need to solve. And these other problems aren't quite as easy to solve.
There are other methods to convert cash into electronic form.
Countries:
USA
Europe
Brazil
Bolivia
In the USA, of course, there is BitFloor which accepts deposits at Chase bank and Wells Fargo. There are no fees, though there are daily AML limits. An additional method includes FastCash4Bitcoins.com (deposit cash at Bank of America,
costs about 1% currently you can even buy at a 1% discount). Get-Bitcoin and QuickBitcoins.net accept cash-by-mail. Most remaining methods like BitInstant, AurumXChange and others use TrustCash to accept cash at major banks. There is a fee for this, under 4% for larger amounts.
You would hit daily limits even using a combination of all of these well before you even start knocking away at your $200K buying intention.
In Europe, there are currently fewer options for turning cash into electronic form. There is Bitcoin Nordic which accepts cash-by-mail (USD, EUR, and other currencies) . There is the option to buy UKash vouchers, and to redeem those through a CashU online account which can then be used to purchase bitcoins -- also at Bitcoin Nordic and at MercaBit.eu as well. That is an expensive method and there are daily limits as well.
In Brazil, cash deposits are accepted at Banco Recomendito and using Boletto (via BitInstant, QuickBitcoins.net, and others). Daily limits apply here as well. UKash vouchers can also be purchased in Brazil and redeemed though CashU for funding at Bitcoin Nordic or redeemed directly through MercaBit.eu
Bolivia too is another place where UKash vouchers can be purchased and redeemed through CashU with funds used to purchase bitcoins through Bitcoin Nordic or redeemed directly through MercaBit.eu.
While there are quite a number of individual methods to move smaller amounts of cash to bitcoins, they do not combine easily to allow fast conversion of such a large amount.
There are likely other approaches. For instance, offering to provide bitcoin cash-out service will yield bitcoins in exchange for an outlay of cash.
Here's an example of a person trading bitcoins for cash in which the cash is then sent by mail or deposited into a bank account. You might consider offering that service. Even if this is done for small amounts per-trade though, there still might be restrictions on offering this though, depending on your jurisdiction.
Now this may seem like quite a hassle, but consider how a about a year ago just to do this type of cash buy with just $20K would have been even more of a hassle. The options available are developing and improving continuously.
There's probably an approach that works uniquely well in each geographic location, but the methods all have a common denominator (bitcoin traded for something of value) and this exchange can complete without needing services from the banking system.
