For me, it would be a case by case. But rust would really turn me off, but if the price was right (Almost free), I might take the gamble. Live and learn.
If you take it home and find out it's junk or unshootable, you can always call the guy and let him know, and see what he does about it, but, you did make the deal as is, so I wouldn't expect him to offer your money back.
If he's a dealer, maybe if you still trust him, he might give you a good deal in the future, because buying a used gun is alway a gamble no matter what.
My brother, got the wrong used gun from an order that was a better deal, he called the guy up, and he sold my brother the right gun for about half price. My bro scored both ways. Of course these were Police trade in's, so I'm sure the guy on the other end didn't lose. But two for one?