I have also had really good luck buying knives on eBay (knock on wood). I do enjoy the fact that eBay's buyers' protections are strong. I also do my research and purchase only from sellers who have a feedback rating of 99.8% or better. Anything less and it's not worth my time.
Like some others here, I don't buy used knives. The one occasion I did, the knife came not as described (big shock), but I was able to rescue it. So, I ended up not returning it, and left the seller neutral feedback, which he didn't appreciate. He sent me an email trying to ream me out, so I sent him several pics (my camera is apparently much better than his) showing him where his description had been flat out BS. I then showed him the knife after I'd cleaned it thoroughly, sharpened it up and restored the edge and told him that the way the knife had cleaned up was the ONLY reason I wasn't reporting him to eBay as being a fraudulent seller, and demanded that he start paying better attention to his stuff. Then, put him on my block list.
The knife in question was a TOPS BOB. When it arrived, the handle was dirty and stained and the edge was tarnished and scratched. The worst offense? The knife was dull. I was livid, but decided to give it some TLC and see how it responded. As I was pleased with how well it cleaned and sharpened up, and it really is a great knife, I kept it. Still have it, in fact.