Caveat emptor ... AKA buyer beware. Good that you recognize this even if you were or were not aware of this exact term.
I suspect that quite a few people selling on Ebay, Craigslist, and now Facebook Marketplace are... if not out and out criminals/conartists ... are pretty shady and irresponsible. Learning these lessons the hard way, maybe you spend less time fishing these murky and toxic sale waters for any allegedly legitimate deals.
Finding a legitimate seller whose selling exactly what they're claiming to sell (sans ulterior motive) is pretty rare on these sites. Not as rare as ... winning the lottery I suppose but not really worthy of your time in the long run.