perhaps even the lenders should prove themselves by signing the address that has the bitcoins to lend.
these threads are pointless. no one has collateral. ever.
Remember even the most reputable members can scam when they are in desperate need of money. I would like to suggest that you require collateral every single loan.
If he is offering loans, the only thing he needs to prove is to provide the said amount of loan when someone comes in asking for loan with right amount collateral and an escrow.
Advise to the person taking loans is don't send collateral directly to him, always use escrow.