FL Vet - I'm not sure that there is a direct way to see a seller's closed auctions. However, if you are following S&W revolvers for example, there are several sellers who stand out as getting stupidly high prices on a consistent basis and the bidding just looks wrong.
The best way to avoid over-paying is discipline. Set a ceiling and stick with it.
To see past auction results, hit the "Advanced" tab over on the right. Then at the top of the box, select the "Completed Items" tab. Set your filters including "Sort By" highest # of bids first, and go. BTW you'll find what I'm talking about relating to shill bidding if you look at enough closed auctions.