Well, just behave absolutely in the same way as in the real life:
In order to get money, the person you are providing it to, has to have a good credit history, returning tc could be done automaticly, in case he or she doesn`t have money to return he or she could be banned or lose all the items as a punishment, I suppose this is quite fair. Anyways, scammers will totally not be 100% of all the people in toribash, so, banks will totally get profit anyways in the end.
There also could be something like pledge system involved or could be guarantors that a debtor has to provide to get a credit, when a debtor not paying tc he took, you took them from a guarantor, for example.

Btw, there could also be also toribash stock markets then after creating several banks, where people can buy stocks of different banks, etc.