I buy guns new only.
But I sell alot of 'used' parts, which for the most part aren't used at all. They are taken off the day the gun arrived to be replaced by mod parts or kits. I only sell because it's a nessesity to offset the cost of $1000+ guns. Otherwise I'd keep the parts.
Generally, I don't buy used guns because of the fact I don't know how the gun was treated or who had their hands in it. I've fixed plenty of 'headache' guns modified by other people and now refuse to even look at another upgraded gun because of the fucked up shit I've seen. My suggestion is to bring it back and demand it gets fixed by the person who screwed it up in the first place.
When the going gets tough... the tough go cyclic.