I'm verified on all three of these services and have found out that CampBX and Bitstamp don't actually buy your bitcoin. You put a trade out there and hope someone buys it. I've always used COINBASE previously where COINBASE themselves would buy whatever I was selling. For those that use CampBX and Bitstamp when you want to sell bitcoin and make a sell order, is it sold up quickly or do you have to wait a while? I'm just talking if I were to use the average going rate at that time. I may want to sell some bitcoin I have and have always used COINBASE but wouldn't mind trying another service. I know they all have fees....and I don't use COINBASE a lot, so I'm looking for experts out there that are familiar with each service and can pin point the differences easily. I may try to sell just a little at each place to try it out just for the heck of it. I've heard bad news about all three services, so who knows.
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.