I believe the seller can cancel someone's bid at their discretion.
Not the same as a bid retraction. There are people out there that get paid to run scams for others on their auctions. They run until someone catches on and reports them, then they come back with a new profile. I personally would cancel any bidder that showed up on my stuff with more than 1 or 2, just to keep from having to allow it later anyway. I can tell you, that high occurrence of retraction is abnormal. Why would that many people allow someone out of a deal in the first place? Either it's a scam, or they are threatening seriously bad feedback? That would be grounds for filing a complaint, and in some states it could even end up in criminal prosecution. That would be considered extortion in my opine.
Sorry, I did not see the post by kevins. I get the whole thing. That is correct, you kill 'em before they mess up your deal, and then you got none, or you get more from the second chancer's, even tho I find that a slimy way to up the value of the sale. Personally, I would have to take the bidder that was highest st the time the "shill" entered and try to work something out, if I were stuck with the "shill" at any point.