I've bought all sorts of used stuff from estate auctions, the Gunboards.com Trader, and gunbroker. If there's some worry it could've suffered damage and there isn't the usual "absolutely fine condition, some honest wear from use" disclaimer, I might ask first if it's an unfamiliar seller, but otherwise... Parts are parts, and guns are durable goods. It's hard to fool an educated buyer and harder yet to sell junk and get away with it. Besides, usually somebody sells because either they need the money or don't like what they had, both of which are good for the buyer.