We have twice sold items on ebay never bought anything. I'm not much of a materialist we tend to buy things 2nd hand locally. I did bid on something, once. I found bidding very stressful. Definitely decide your top price and do not go above it.
My tips to add to Angiel's: be prepared, if you're selling, that you may not get the price you really wanted.
Make it clear if postage is included in the selling price, or how much it is.
It seems like most people just trust each other to send cheques and the goods when the cheque clears.
It took us DAYS to get our first ad up. And DH & I are both computer geeks!
Take a look at lots of pages to see what an attractive advert looks like. I don't think it's necessary to pay for the extra front page listing, and all that, but the nicer your advert page is, the better the price you'll receive (honest!).
Certain items will always sell better on ebay than anywhere else; but some items sell lousy on ebay. Electronic goods go for really high prices. I am thinking to flog a buggy board on ebay; I think I'll get a better price for my wedding dress if I sell it locally, though.