It would be abused and have one more thing mods/admins have to do.   And I think it would be all for nothing.  Even being verified people are willing to scam/default BTCJam is a perfect example of that.  Some just don't care what their online name brings up.   So verifying a scammer.... he is still a scammer. 

Also photoshop will make this pretty hard as people will submit fakes. And what if a verified user defaults or does not provide the item sold?   It is still going to be hard to get money back.

The trust system is best we can really have. It allows you to track a user if they have done deals in past, and escrow.  I think escrow beat's this system by far it put's a known middleman to make sure deal is real.   So I think with trust and escrow all the tools are already there we need to have successful trades.  Getting verified sounds like a lot of work for very little actual gain.