Like everyone else here, I've plenty of complaints to make about Ebay as both a regular buyer and an occasional seller. In my case over the course of twenty odd years--with many of them growing far worse when masking bidder identities began (first during auctions, and more recently even in feedback). But then also when--in many fields (with estate pipes somewhat of an exception)--sales began to edge out auctions even with unique vintage goods.
My inference back before either was the case (which is say auctions were the norm and you could keep track of the whales of whatever collector world you were operating) was that--as Jesse suggests--retractions were most often a strategy used to get information.
Typically to figure out a seller's reserve price (back when such auctions were quite common and when sellers' reserves could be quite high). The other situation occurred before sniping became quite so prevalent, and at least some experienced bidders preferred to use Ebay's proxy bidding feature, and place a strong bid early in an auction. Occasionally, let's call it malicious curiosity, would get the better of someone with some interest in what was on offer, and they'd use Ebay's bid retraction feature to find out exactly how strong their rival's early bid actually was.
The third situation--random flakes and scammers simply spoiling an ongoing auction for no clear reason (a category that would also include the virtual world's version of kids and their friends making prank calls)--could also be a problem. And of course, there actually were genuine honest mistakes--one zero too many, misreading an item description (or failing to see a serious condition issue in a photo), or not noting that shipping cost from Shanghai was going to run $100.
As a seller, I always did my best to guard against situation three using Ebay's "bid cancellation" feature. I would always make a point in my listings to warn potential bidders that new ebay buyers, those with poor feedback and anyone from outside the US (to which shipping costs could be significant) needed to contact me BEFORE bidding. And THEN I'd watch bids as they came in, and check out the bidder's history to assure myself as much as I could that they had some history of actual honest dealing. And perhaps on a couple of occasions I actually did cancel a bid.
Finally, I also made it a strong point of encouraging potential bidders to contact me with any questions they might have. (Whether on Ebay or anywhere else, whether as a seller or buyer, my hope is to encounter a reasonable person on the other side of the transaction.) To some extent, I'm inclined to think, limited as they were, such preventative measures may have helped. As clearly did a certain amount of good luck (with perhaps a couple of hundred sales transactions).
I have to say my guess, joeman, is you've just had a run of bad luck with category 3.
(As for the possibility of some scammer approaching a seller with a low ball offer after a failed auction--Ebay's rules makes that tricky at best, and virtually impossible if you wanted to deny them their cut.) As for ebay itself creating shill bidding bots. . . I really think not.
Like all large and successful auction venues,Ebay thrives on a strategy of sitting happily in the middle and extracting their £s of flesh with a thousand cuts from both sides. This in the confidence that sellers will believe they'll do a bit better with them than they will with a sale to a retailer (straight up or "on consignment"), while buyers are generally looking for bargains (and more than happy to deal with sellers who have no clue whatever what it is they're selling). Which is to say, it is well to remember that even with many honest potential buyers there is a somewhat adversarial element in the mix. And plenty of people making perfectly real bids (who'll cheerfully convey payment, readily acknowledge the arrival of a package, and leave you glowing praise as feedback) are out to score a $500 pipe for $50.
Very much my 20,000 cents. But hoping some part of it might be vaguely helpful.
