Mechanic here btw. It's not that its a scam, but it can be abused as such like many things. If you're looking into selling your used auto parts because you think you can get some money for it. Avoid the brand shops like Autozone and O'reillys, or for some god forsaken reason if a dealership is doing it, lol stay away. I like to sell my used parts if I think they can get any use on FBmarket, or sometimes i'll do the core charge thru something like rockauto because I've never had a problem with them as a customer, but its usage is quite rare. In general I prefer to keep the middle man out and support local if I can, and you should too.
edit add: Also junkyards are a decent to throw your stuff if they'll let you, or find your local public landfill/hazardous waste center, I go to mine to dump used oils and stuffty broken parts.