This is why I no longer cruise the gunshows. They are the perfect dumping ground for damaged or malfunctioning firearms.
That's why you bring a good mini flashlight, bore light, feeler gauges and a few extra zip ties so they cant use the
"I cant cut the strap because I dont have any zip ties to tie it back up" excuse.
If you properly inspect a gun prior to purchase, things like cracked forcing cones, loose lockup, end shake, bent extractor rods, bent or missing(!) firing pins, bent hammers, incorrect timing, canted barrels, bulging barrels, rusty or fouled barrels, cracked frames, cracked or damaged cylinders, etc, etc. can usually be found. And then you can have fun bargaining with the dealer or walking away, depending on what the defect is. If it was me, and I found something that I knew could be fixed myself cheaply, I would point out the flaw. Chances are, he knows it's there anyway and cant hide/fix it. You can use that as leverage against him. You can even say if he wont deal with you, you will stand beside his table pointing out he is selling broken guns, maybe even report him to the ATF or state regulating authority.
Case in point, I found a pretty decent 28 no dash 6" at a pawn shop. It had a broken rear site leaf (that I know I can fix cheaply), and was dirty to the point the cylinder would not eject cleanly (again, fixable), I bargained him down so low, I am ashamed to mention the price I actually got it for. I think once I pointed this stuff out, he was happy to get it out of his store.
I took it home, cleaned and lubed it thoroughly inside and out (was FILTHY) and it is now in pristine shooting shape (with the exception of the rear site, parts on order). I got an absolute steal.