This is your mistake. As I said above, just because a member is part of a signature campaign doesn't mean he can be trusted. You should always keep this in mind! You have been here for too long and am sure you remember the case whan many high ranked/trusted members were busted for being alt accounts. Some of them were part of the highest paying sig campaign, back in the time.
Anyway, I hope some of the defaulters will have some integrity and repay what you owe them.
My observation is that no one with a single account will default such a small amount which is evidence of the defaulter. But it is not an easy way to find out who has an alt account and who has not. On the forum, we can't nothing judge which can save us. I have seen two users defaulted on the loan of Direwolf who has good rank as well as merit ranking but defaulted. So, we have nothing except to accept this type of loss until there are alternative accounts that are decreasing by being banned and defaulting on loan.