This might be a little obvious, but when you view a listing and the pictures are out of focus, be very cautious.
I once bought a used CPU for my computer where the picture was out of focus. When it arrived, it was clearly damaged, in contrast to the description, which stated that it was in perfect working order. The die was chipped and I had to straighten the pins. It then didn't work when I tested it. I'm no slouch with a PC, as I have built a good number over the years without any problems so I was certain it was faulty.
When I complained, the seller told me that it was fine when he sent it, so I must have damaged it. Further correspondence descended into personal atacks and threats, which did not phase me, and I put in a dispute with ebay/paypal and a complaint about the abuse.
End result was I won eventually, but it was rather a pyrrhic victory, as I got my refund, but only after I'd had to get an independant report at a cost approaching half the original price I paid! The other person was off ebay shortly after.
If the pic is fuzzy, ask for a clearer one, or walk away!