[rjmunro]

> If there's no limit, no "credit limit",

There is a limit. It's the limit set by your friends.

> If there's no limitation on what constitutes an identity, then what prevents someone from signing up multiple times as different users

Nothing, but it doesn't matter:

> .. and giving themselves extra "credit" (ability to take) and taking, that way?

They'd only be able to give extra credit to themselves. No one would give credit to any of the extra users they have created, so the extra users would not be able to pay anyone anything.