Exchanges (such as Bitstamp) and sites that buy/sell Bitcoin (such as Coinbase) function in quite different ways. Coinbase just provides you with a price to buy at (or sell at) and then it's take it or leave it.
On Bitstamp you can provide your own price and if there are people on the other side of the offer that have the same price in mind, a trade is made. If there are sell orders for $820 in the books, then my buy order at $820 will be executed immediately. However, if I place a buy order at $810, I have to wait for the price to drop and for people willing to sell at that price to place their orders.
Well I'm already having problems with CampBX. Very first time I sold a little bitcoin and the transaction was done in like minutes but the money is now stuck in limbo and hasn't been deposited into my bank account. I thought it was illegal to just hold onto people's money and then not pay them back without some kind of interest on it? The transaction was January 22nd and today is January 28th. You'd think on the very first time you use the service things would work, but this is an easy decision...never to use these guys again.