Ebay.co.uk is a great site. Its fees are more or less reasonable and you get many more bids in your products than using amazon.co.uk, for example. The problem is about the way the comments are done: after an auction, the buyer and seller can leave feedback on the others' account. If you think a bit, it is great to be able to check a seller/buyer's profile. The trouble begins when the other part is not honest...
I will tell you about one of my first bad experiences with ebay: It was an auction for a SIM card for a mobile phone and I was in a hurry and I really needed it. Anyway, I thought that if the seller had it on auction it would be because he was willing to sell it. Wrong! I contacted that person more than 4 times (the first one ten minutes before the end of the auction and the last almost a week after) before giving up and buying a sim card directly from one2one. Then I learned he had changed his ISP (and his email, by the way) and hadn't updated his contact details in eBay. I have to say that after three working days if you are not contacted by the seller, you can reject your bid/contract, but I politely replied to this person, letting he know I had been waiting for him to contact me and I had to give up, *but* that I was willing to try to find another buyer for him if he let some days. His answer was just an insult. So, I ignored him. (I didn't even think about letting any feedback)
After some weeks, I had started to sell some items and my feedback was great. But one day that seller -probably after feeling envious of my profile- decided to spoil me and wrote some lies, saying that I haven't ever contacted him, etc. You can't do too much in this situations. You can comment on any feedback left, but you are limited to one sentence.
Another big problem is about bidding in US auctions: you must be very careful with your email address. Why? Because there are some people out there that put something in auction just to get a list of selected email addresses. For example, I bidded in an original copy of windows 2000 professional and no bidder was unsuccessful because the high reserve price. After that I've been receiving almost every day junk emails about cheapCD-R backups, pirate software, ... Peculiarly it started just after the event I mentioned above.