Firstly, it was not sold via the forum, so the forum will have nothing to do with the dispute on an official basis.
Secondly, I know Paul, and know that if the box was not in full working order he would not have been happy with it being sold without telling you of any problems. Something like this is the last thing he would want coming back on the company.
Thirdly, my first thought would be that the buyer has removed his damaged gearbox, replaced it with your working box, and may be trying to claim that his damaged box is the one you sold. Therefore, getting a working gearbox for free, and palming his damaged one back to you. (I do not know who the buyer was, and this comment is by no means an attack or accusation)
Anything like a gearbox, I would always mark it somewhere subtle, so that if anything like this was to occur, I would be able to identify the box as being the one I sold.