Treat it like eBay. I'm sure there are some bad apples on there, but if you check their feedback before accepting a bid you shouldn't have much of an issue. Plus, you don't give the funds directly to the shippers... once you accept a bid, you put the funds into uShip, who then gives the shipper notification that the money exists and gives you a randomized code which you only release to the shipper once the item has been delivered and you are satisfied that it wasn't damaged or anything. Detailed pictures are taken before shipping to prove condition in case any damage occurs in transit.
I was happy enough with my experience that I'd use it again.