Look first for signs of neglect. A POS is not necessarily rejected out of hand unless the seller is trying to get "antique" prices for it. The biggest issue is rust, and more specifically, rust in the bore. Hmmm.... seems like I've read that before somewhere.
Even a badly rusted bore isn't necessarily grounds for rejection... but it IS a concern, a major concern. I have a T/C Hawken in .45 caliber that had a pretty rusty bore when I bought it. The stock was in good shape, though and I had another T/C Hawken in the store with a good bore, but a cracked stock. The crack was so bad in fact, that the stock broke in half in my hands.
I swapped out the barrel with the good bore and mated it to the good stock, then put it out for sale. Looking at the rusty .45 and the broken stock, I thought about it a few days while it sat on the back bench. Eventually I bought it. The stock was repaired with epoxy and has held up well over five years now. The brass polished up nicely with Brasso. I replaced a missing ferule with another one from a T/C Greyhawk. It also got a new nipple. That gun is referred to as either "The Monster" or "Frankenstein", it is pretty much my favorite gun... though it is hard to choose my absolute favorite.
I worked the bore with a stainless brush and Murphy's Oil Soap/water. Later on swapped the brush for a fitted jag and some fine steel wool. I can't say that the bore is pristine, but it is usable and shoots acceptable groups..
I think I have maybe $150.00 in it total, plus a fair amount of elbow grease.
Bottom line: if, after buying the POS and putting it into good working order, you have less invested than you would have by buying a gun in better shape, you did OK, else, you shoulda kept your money.
You can buy replacement barrels and you can have barrels re-bored to a larger caliber, so even a really bad bore is not an absolute disqualifier. Think with your wallet and don't fall in love with the first gun you pick up.