Rollers are fun. Offer him $50 and then let him talk you up to $80. Have the cash in your hand. If they're old and rusty, he probably just wants to get rid of them, and you'll be doing him a favor by making room in his garage.
If they've been sitting around long enough, the belt is probably getting ready to break. You can get new ones from Riverfront Cycle in Lansing, MI (they still make McLain stuff) for around $15.