I will almost never buy a used gun, unless it is guaranteed bone stock, functional, and unpainted. Even then, I am weary. Half of the time "upgraded" used guns are worse than stock, because most of the time people have no idea what they're doing. I got a gun off ASC a few months ago, I didn't think there was anything wrong with it until I sold it to BioRage up there. He messaged me later saying the HOP UP RUBBER WAS FUCKING MELTED.
I've bought used "upgraded" guns from the classifieds when I was new. They all called themselves gun doctors or took them to reputable ones. What I found was shit like a completely rusted inner barrel, cracks in the receiver, misfit screws, fucked up internals... You name it. To a noob, the sellers looked pretty reputable too. One of them had over 100 positive ratings.
I have gotten some pretty good deals from the classifieds, so I am still open to buying used guns occasionally, but it must be guaranteed bone stock and un-fucked. As far as "upgraded" guns go, I will only buy them from a builder I trust, of which there are very few. One guy offered a gun to me as a trade, it was an instant bag of NOPE from me. It was built by a guy that some seem to think is reputable, and would sound like an amazing build to people that are clueless. Just looking at the posted performance specs I could tell that something was wrong with the shimming, something was wrong with the air seal, and that a gigantic hole was drilled in the stock to install the MOSFET.
Guardians of Asgaard - KF32
VFC H&K 416C
VFC KAC SR16 JL Billet SBR
TM FN Five-seveN