There are sellers that will do that. Once upon a time
ebay let us see who we were bidding against and we could get a better idea that we were dealing with sellers that do this. They stopped this when they thought people were making too many side deals and cutting
ebay out of their commission. It was not unusual to get emails from guys after you lost an auction offering to sell you a similar stamp.You can report your suspicion to
ebay, but if he is a power seller generating income for
ebay it is unlikely they will look into it.
ebay allows multiple ID's. I have one I use for buying and another for selling. I know of a seller that would cancel auctions on the last day on a regular basis if the bids were not to his liking. He would tell
ebay that that the stamp was lost or damaged. He did this so often
ebay restricted his ability to do that any more. I would not be suprised if he now has multiple ID's or friends bidding his stuff up, although I have no proof of that. He is a shooting star power seller and can get away with a lot. I won't bid on his auctions anymore and I won't identify him either. My best advise is to look at the feedback and look to see how many auctions were ended early. If either look out of line, don't bid.