Used Nelson
In my opinion, there are very few "good" used Nelson engines available. Most of the New Engines are very close in performance, but the used ones tend to be a little off.
I know if I wanted to unload one of my used nelsons, I obviously wouldn't get rid of my fastest ones. Also, if I had a Used engine that I knew was a horse, I would sell it to one of my buddies.
I have a used Nelson engine with about 50 flights on it that I would be willing to sell (less than 2 years old). However, it turns about 500 less RPMs than my good ones. I have messed with the liner and head spacing, but it still just doesn't turn up like my good ones.
I have a feeling that this is the type of engine you would receive if you tried to buy a used engine. I doubt you will find the horse that you are looking for through this route.