Here's an example of a seller where I feel they crossed the line in terms of violating eBay's contracts and policies: (insert name of seller here).
For years (insert name of seller here) would list very large lots of comics, using big photos where all the comics were spread out over a wall display. A lot of people here know what I am talking about. They would often list them as auctions with very high reserves -- I never experimented to see how high the reserves went, but people would bid up the auctions into several thousand dollars territory and they still wouldn't hit the reserve, so most bidders gave up. I know I did.
A friend of mine tried to bid on some of their stuff and didn't hit the reserve. Then, after the auction ended, (insert name of seller here) sent that friend a message offering to negotiate a sale outside of eBay. They worked on a price and finally my friend bought the lot -- it was something like 100 or more EC comics in all genres. My friend told me that the average grade/condition listed by (insert name of seller here) was way off and most of the comics were much lower grade, and I think he even complained to (insert name of seller here) but there wasn't much he could do because he wasn't protected by eBay policy. (insert name of seller here) might have shaved a little off the deal to appease him, I can't remember, but my friend was very disappointed with the experience and ended up spending months auctioning off most of the comics individually only to just about break even. (This of course was a valuable learning experience for a buyer...)
I also bid on a few (insert name of seller here) lots. I knew the drill and that I wouldn't hit the reserve, but there were some lots that had really cool stuff and I threw in bids for the heck of it. Several times, I ended up getting post-auction messages from (insert name of seller here) offering to make a deal off-site. After a few of these encounters, I decided not to bid on their stuff anymore. They had rigged the game so that it would always come out in their favor.
I think it's fair to conclude that (insert name of seller here) was using eBay entirely as a marketing site. Their entire setup was designed to avoid actual eBay sales, and therefore avoid paying percentage fees on transactions. That's just shady.
Once somebody reveals that they're OK with one form of shady practice, the obvious next question is: What other questionable practices are they capable of? This question applies to a lot of other sellers and venues I've experienced or read about.