On the Bitstamp platform, if I click buy, does my "buy" request go on the order book waiting for someone to "sell"? or do you I need a special account to enter orders in the order book?
As long as you are executing limit orders you should be able to place orders on the order book.
On bitstamp, there is bid/ask. Bid is people giving you fiat for your btc. Ask is people selling btc for fiat. When you click buy, you are essentially placing an order on the bid side of the book/or filling someone else's order if the price is right. Vice versa with sell.
There is really no difference between setting a price at the highest ask and setting a price below the highest ask price. In both cases you are directly dealing with the orderbook. Purely preferential as you may get better rates if you choose to wait a bit. There is no special account to do this whatsoever.
