as someone noted the seller won't make many friends if he is bidding against his customers. We had a very good antique dealer here in Syracuse in the '70's who told me he rarely went to local auctions since he did not want to appear to bid against his customers. At a later auction I watched him bid up a nice piece my then wife wanted even tho we had bought from him in the past. The flip side of the coin is that when I attended auctions and watched the dealers bid, I knew when they stopped what the wholesale was on items and this helped me determine how far to go on something I or my wife wanted.