It isn't that much different from "real-world". There are tons of services where you pay in advance because you trust the other party as you know that it would hurt their reputation too much too just walk away with the customer's money.

This works even better in case of a repeated contract: For example if an online service with a subscription of 3$/month suddenly stops giving value for your money, you just stop renewing and they loose a customer. You know that, they know that, they know that you know it... etc ad infinitum. And things keep going smoothly.

I am not saying that what I wrote solves all the problems (one-time contracts such as ebay is a different story), but this argument itself does not prove that bitcoins are worthless.