It's funny that the only people who give Proxibid a good rating are the third-party auctioneers. Their business is based to make THEM happy. The auction houses are their customers. If they don't get them to sign-up and sell through them, they have nothing to sell.
I had a very bad experience with Proxibid and I believe they took the side of their auctioneers over me. The auction house wrote their "rules" in the auction information and did a complete 180 from what they wrote. Having being charged an additional fee I did not agree to, they chose the auctioneers side, basically saying "that's the way it works." Just beware, if you're going to use Proxibid you may be on your own if anything should arise.
I will never use their service again.