Regarding problematic guns, here are the sum total of the "problems" I've had with my guns:
New Remington 870 that wouldn't extract reliably - The channel where the extractor spring & follower reside was gunked up and needed cleaning.
New Walther P22 spitting spent cases at my forehead - new extractor from Volquartsen fixed the problem.
New Springfield EMP that wouldn't lock the slide back on an empty magazine - Springfield sent me a new slide stop pin with a slightly different profile that fixed the problem completely.
80-year old S&W pre-Model 10 wouldn't allow me open the cylinder - the threaded rod that screws into the extractor star had come unscrewed.
That's it (although I've only owned about a dozen guns).
My point is, guns are not extremely complex, so unless there's an inherent flaw in the design, or catastrophic damage, is it really that difficult/expensive is it to get one running right, as opposed to selling it off at a loss or as a "parts gun"? I guess parts availability could also be a problem with older guns, though.