Not to mention campbx needs you to pick up a postal money or just mail a check (ach to come in two days). And coinbase, you can sell at today's price, but if you buy you have no idea what you're going to fill at, 4 days from now.
Basically liquidity is not your friend here. If you could wait for your money, though, you'd make tremendous returns buying at campbx or coinbase, and selling at mtgox, but from the sounds if it you could be waiting quite a while before the funds reach your account so you can trade again...
When you buy on Coinbase you lock in the price when you place the order.