will just say with a used nitro, plan on buying a new engine.
Pretend the seller had no idea what he was doing and toasted(chances are he did even if it says runs great) it and pay accordingly.

Thing with nitros is that if the sleeve and piston are shot they will run for 5-10 minutes. Pretty much every used engine I've got in a used nitro either the sleeve and piston were worn out completely, or the prior owner ran it insanely lean and royally screwed it up.