I had a seller refuse to sell me something unless I paid £40 (I won at £5) and Ebay apologised but said they couldn't force her. I left a neg just stating "demanded £40 when won @ £5" and she immediately negged me back saying I was lying and had made threats! I was as hell. I still can't believe she had the nerve to do that, and then screw my perfect f'b over afterwards! Ebay brought in a ban on retaliatory negs shortly after.
I'm also currently in a dispute with a guy who sold me a Folio Society edition of a book I want as a b'day present for a friend, that the FS are out of. Used copies were going for £3, but he said his was new and he's a dealer so I assumed probably more reputable than an individual, so I BIN for £21 plus P & P.
It arrived filthy, spine tobacco yellowed, stinking of smoke, and he ignored my email to say as much and ask for a return & refund. In response to my Paypal claim, he's said I can have... a tenner. Um - no! Why should I pay £15 for a book I don't want and is worth almost nothing, especially as I'm bidding on another new (hopefully!) copy, and I'd already offered to pay the return postage on the assumption it was a genuine mistake? All I did was bid, on his own description, in good faith! Grrrr!
Fortunately, most sellers are absolutely lovely. I bought a book this week for next to nothing, and the seller used shiny paper so the stamp fell off and we got surcharged. I sent him a message saying not a problem as it was a bargain anyway and an obvious error, and it was only a couple of quid on the postage so I wouldn't leave bad f'back or anything, but just so he knew, in case it happened again and another buyer was a pain about it. He promptly gave me a full refund, unasked, and lovely f'b!