What would they be worth?

After looking them over, if you feel good about it offer him 5 for the pair- if something proves to be amiss, you still can recoup your money in parts. Otherwise, it's a stretch for unproven condition but still not insulting to your friend. BUT- I would first see if you can get him to throw a number at you just to make sure you're both in the same ballpark. Maybe there's a few other nuggets in the trailer to sweeten the deal, too.