Originally Posted by MarkCO
As long as the frame, barrel and slide are not cracked or otherwise damaged, you will be fine. I know a guy who went to a GSSF match with a Glock he got taken on used that had no trigger guts or housing, just a paperclip holding the trigger in place. Armorer put ALL new parts in and sent him off to shoot the match.
Look for hogged out pin holes in the frame. I press them out and put them back to make sure everything is correct. Dirty does not matter much to me, means they are not trying to cover somehting up (usually) and shot it. Run your pinched fingers down the barrel feeling for bulges, and then look for any rings or pitting IN the barrel. Make sure the extractor is not broken (if it is, bargain the price down and still get it). I don't care a bit about hand cycling rounds, gives no indication of proper function, so shoot it if you can meet seller at the range.
My philosophy. Make sure the gun isn't stolen or anything, matching serial numbers, and as long as the slide, barrel, and frame are in good condition I'll buy it, but not for more than $400 because if I have to replace internals anything over $400 I might've been better off buying a new one. The pin holes in the frame is a good suggestion, some people detail strip so often that these holes get too enlarged.