To be honest, you shouldn't be taking an account as a collateral because that gives defaulter an easy to sell their accounts. Just in case if he defaults the loan, what would you do with the account? Probably sell it to get whatever you can to cover your loss? Sounds like the root cause of account trading?
I just held onto mine (when I used to do it).
It's not much of a collateral that you take but just some sort of incentive to get people to pay back.
He hasn't been online for a good couple of months other than to post the loan listing, I'd quit this one mday if I were you...