You are going to lose money if you keep doing this! Ask for collateral!
Yeah or not at least approving any requests from accounts with no trade history and below Sr. Member. I think signed message is fine as long as the account is something that you would see the person coming for, i would not see HumberDice for example coming back for his, because of negative feedback and accused multiple alts.
But i agree with this reply, OP you might want to consider more strict lending.
Yes. That site is really not accurate at all so you should not trust it. I would suggest just evaluating every person, reviewing their trust and post quality. You will not want accounts as collateral which have no chance of getting into sig campaign or are otherwise horrible quality.
Edit: also if you still want to take a leap of faith with signed message, make sure it is from staked address so it is not hacked account. If you for example look at HumberDice's post history closer, you'll see that it mostly is just spamming to farm post count.