Easy solution - ask for the VIN.

Needing to have the tranny pulled and opened up is going to be something too expensive to have fixed, and if you - as the seller (or imagined buyers) aren't someone willing or able to DIY, then it makes the bike worthless to be sold, hence the free. They are asking to have the issue removed at no cost to them, on an item that no longer holds pleasure for them, and that has probably cost a pretty penny to them at this point having it worked on by a shop to even get to where they know what the issue is.

Maybe they bought it in late '83 as an '84 build, and don't know it's an '84. Or it was sold to them as an '83 and they didn't know how to check.