What would be some of the causes for concerns, leading to a lemon posting? What would be the culprit for issues with burning pan powder or ignition in general?
Gene, are you asking how you could know a rifle for sale is a “lemon”? Specifically, in functioning poorly or being inaccurate?
If a gun is a build of a high quality kit, you know it has good components and is unlikely to need more than a little fiddling and getting used to. If a used gun or a scratch build, if made by a reputable builder, and well maintained, it should be good. Outside of that, the things to be sure of are the barrel and the lock. If commonly viewed as excellent, those are 2 less things to worry about.
I’d not advise buying a gun of unknown origin as a shooter unless it was coming from someone with a trustworthy reputation who is also a shooter who insists on good quality for their guns.
A great many functional issues can arise. Slow locks with poor ignition of prime. Poor touchhole placement. Poor trigger function. Barrels with issues.