Hi, I'm quite new to bitcoin, so new that the sh**** transaction blocks haven't downloaded and verified. I have a couple of questions:

1) Anyway how come the "I-buy-bitcoin-from-a-dealer" price at localbitcoins is sometimes LOWER than the "I-sell-bicoin-to-a-person"? BTW usually the bitcoin buyers have better reputation than the low-price bitcoin-sellers, but assuming it isn't a sort of scam. You could have money instantly without an effort, or couldn't you?
2) BUT  when you buy 1 bitcoin from some seller (perhaps not localbitcoins, it may be another "service"), you spent some money, which can have a long road ahead and meanwhile the price of bitcoin goes higher, does the seller send you 1 bitcoin or less? (and when the price lowers he could give you more than 1 bitcoin ?).
3) What defines volume of you bitcoin wallet? Transactions stored on every bitcoin-qt equipped computer or what? (I don't completely understand http://bitcoin.org/bitcoin.pdf part 7 Reclaiming Disk Space)Then in the future (or is it already now?) the old blocks are somehow "cropped" that they have smaller size and bitcoin clients have some ?checkpoints?.

a non economist question:
4) Where does the Bitcoin value come from? Somebody (Satoshi) has just invented a currency and now there are "redundant" billions of i.g. dollars?