I appreciate entirely where you're coming from, Mike.
If he's a nice chap you could ask him to whip the stocks off them and have a look at the cocking arm / underside of the cylinder to assess how much, if any, scoring has occurred. You never know, you might get one with no marking, as it seems they don't all do it. Maybe then choose the one that you most fancy / has the least scoring. If wear is evident, it may also prove to be a good bargaining point.
They're dead easy to strip and you'll have loads of fun and derive great satisfaction doing it.