I think djlaw has it exactly right.

Keep in mind a few realities:

First of all, the market for a product like this is very small. Most people don't really understand how miles work, and they don't seem to be interested enough in them to spend a lot of time trading them.

Second, the purpose of miles (by those who issue them); namely, to encourage brand loyalty. If there was a successful system (and it certainly appears that points.com is not one of those) that was developed, a couple things would/could happen:

1) The airline would change the rules in a manner that we would not appreciate. Think of a rule that said that you could only redeem your miles for yourself and immediate family...or just yourself.

2) The airline gets rid of miles altogether. Think about it. If the means to encourage brand loyalty is not accomplishing that, and is, as djlaw said, completely fungible, the airline has no incentive to issue them any longer.

Finally, consider what you really want to do. You're wanting to buy a sandwich at Subway, and trade your subway club stamps for a quizno's sandwich. It's just counterintuitive.

Be careful what you wish for, you just might get it, to the detriment of all.