Never really had a BAD experience - more like a three odd experiences, all was well in the end. 17 deals. Strangely all involved Marlin Camp Guns.
1) Person (older woman) took forever (months) to ship the gun after I paid good money. Camp45 sold as near new. It was near new, but filthy inside, maybe shot 10 mags (70-80 rnds) of WWB and just put away. NICE gun. She was just slow.
2) Bought a Camp9 from a dealer FFL. Gun was advertised as GOOD shootable with little wear. Pics were too far away to see anything (lesson there!). The gun turned out to be unshootable and the barrel had been spray painted (Krylon) black! I paid a going amount so I was PISSED. I called the guy....he said his guys list the stuff etc (bull, but maybe have some truth, but he is responsible)....anyway he gave me MORE than half my money back - which I was quite happy with AND I got a project gun. Turns out the thing is now very sweet, had the barrel reblued, I replaced springs, buffer, maybe 6 other parts. LIKE new.
3) Two Camp9 mags being sold as Marlin branded mags. Huh? They weren't even S&W branded!! (Mecgar) Guy (dealer) just plain lied. I griped (bad feedback threat) and he sent me FOUR (4) S&W mags.
Everything else has been perfect and I scored some good deals.