And you can buy the 4 bitcoins for sale there at $2.50 or whatever and sell them for $4 profit.
Arbitrage opportunities do exist, but it's hard to really be worth your while in the small Bitcoin economy.
If you don't mind 4 hours of work for $4, there are some sweatshops that might be interested in hiring you.
It doesn't. But it takes time to set up accounts everywhere. It takes time to transfer money. It takes time to *identify* the opportunities. That's the key there.
If it was so easy to make a profit, wouldn't someone already be doing it? My guess is people already are.