I've been selling on eBay for about nine years now, and in all that time i think I have only had about three or four negative encounters, most revolving around bidders not paying for items they had won. While this is a real pain, I have found eBay to be very good at crediting back your fees, so you havent lost any money as such, just time.
Its more an issue when you are the buyer and you do not recieve the goods, although if you pay through Paypal you can make a claim for the full item amount through Paypal. I was on the recieving end of this once, where i shipped an item to France (non recorded post as the buyer didnt elect to pay the additional fee) and it didnt arrive. The buyer issued a claim against me through Paypal and as i couldnt provide a tracking number (despite my original T&C's of sale being for standard delivery) Paypal took the full amount from my account to refund the buyer. While i was annoyed at being on the recieving end, it does provide some security when I'm the one buying to think that there are rules in place to protect the buyer.
If I'm in doubt about buying an item, I tend to send a message to the seller. The response rate and clarity of the response to questions i have answered often provide a good gut feeling, and I keep in mind the old adage of 'if it looks like a bargain, it probably isnt'. Particularly with regards to named brands. The amount of fake goods on eBay is quite astonishing.
Thanks,
Sarah