I've bought nearly my whole diaper stash off eBay. Only once did I get something labeled "EUC" that was less than ideal (fraying, worn edges, velcro was full of lint, and lightly stained). And I've litereally bought a hundred or so diapers off eBay when I had two in diapers for a year and a half.
Just follow the advice people have given so far, ie. only buy items with photos, feel free to email the seller with questions, etc.
You can get some great deals on eBay. Just make sure you read every detail of the auction before bidding. And make sure to check the seller's feedback rating too!
if you are looking for used, also check out the trading post here on mothering! there is also a thursday swap on the babycenter diapering board. i have perused diaperswappers.com as well. i have bought off mothering and bbc and been pleased.