A carney sideshow is a apt description of some of these jokers. We go to a lot of estate auctions locally, and there are one or two auctioneers that we just don't bother with - because they obviously use these shill bidders.
I've never had any of these problems on eBay though. I did tell a fellow off recently when he was 'bragging' about having a friend that did this for him on his eBay items.
I briefly dealt with a jewlery selling.
Magically it appeared he had the same customers who did the shill bidding for him.
It was somewhat obvious with the same "customers" all the time.
It seems if they just use a reserve bid, there is no need for a shill.
Oh, I've seen it rampant on eBay, in certain categories. I had the misfortune of being a seller for a while in a hobby that "took off" when collectibles were the big thing.
I wasn't in to that, had other uses for the "products" - but it was sickening to watch. I didn't pursue it after a few weeks, although I had been counting on the income from selling off what I had collected. Still have all of it, many years later and no way to enjoy it as I once did.