When someone has spent TC, even if it was loaned to them, there is never a guarantee that they will be able to get it back. That's usually how spending money works. Building a system around it wont change that, unless that system creates a whole lot of money to repay loans, or takes money away from other people.
If it creates new TC to repay loans, then it encourages bad loans. Also bans people?? Oh and it needs a much more intelligent system to make sure it's not exploited since it can generate TC (probably just heavy staff moderation).
If it reverses all transactions made with loaned TC then that's going to cause a lot of confusion. You'll have sold an item to a guy who sold an item to another guy who bought that item with loaned TC. A loan that he didn't pay off and got banned for not paying off, then as a result the transactions involving the TC from the loan get reversed so the money comes out of your account and back to the guy who gave out the loan. So you end up with one banned person who took out the loan and didn't pay it off, you and one other confused person who's inventories and balance have just mysteriously changed while transactions were reversed in order for the system to repay a loan, and one happy guy giving out loans without a care in the world who wouldn't even know how much trouble he just caused three other people. And this happy guy giving out loans is now inclined to do it again, because if his loan goes wrong, that's everyone else's problem.
I'm not sure of a third genius scheme. Both of the above have happened before, we already prefer to avoid and prevent these situations as much as we can. Hence "loan at your own risk"