I bought a beat up Marlin Model 60 for 30 bucks at a flea market, took it home and cleaned it up, then traded it at a gun show straight across for a NIB Benjamin pellet rifle.

I also traded a Ruger GP100 that I was into for $230 for a like new Colt Detective Special .38.

I've made other really good trades, but those are the first to come to mind.