Depends what gear you have.
The most usual reason for scrapping a hydro box is wear on the swash plates in the pump or motor.
So if you can polish them flat again there would be a market.
Eaton no longer do mower boxes and there are a lot of them out there, usually found on top end mowers and small compact tractors where the owners are not afraid to open their wallets.
OTOH I do around 200 repairs / year and toss around 4 boxes a year that could be repaired but the owners will not foot the bill.
If you are thinking of doing a rebuild type service, the tractor owners and restorers would be a better market.