I've been both screwed and made deals on eBay. When I built my rig, all stores were out of stock of most of the Opterons (except places that were grossly overpriced). I got my Opteron 146 for $200 on eBay (new), where everywhere else was either $230 or out of stock. They also remained out of stock for 2-3 more weeks before coming back in stock at a much higher price. So I definetly made a good deal there.
Then I bought a Logitech MX700 off eBay. It was a deal so I went for it. I didn't realise anything was wrong at first, but it was burning batteries very quickly. I thought it was the batteries at first, but then I realised the mouse was defective. By then it was too late to do anything about it. I only lost like $20 but, that's $20 I could've put towards a new G7 or something ><.
Anyway, don't buy used products on eBay. Only buy new things if you're sure it's new and they have some sort of warranty / return policy if there's something wrong with it.