not much to go on. is it a manual transmission or the select-o-shift? A manual trans, you can at least pull the top plate and take a peek inside to see whats going on. if its the select-o-shift, those are best left to an expert to repair. Lots of valves, o-rings, and so on in them.
Needing such a repair raises a flag for me. Owner may well be trying to sell because replacing the tractor may be cheaper than the repair itself.