I would keep the tractor. They don't loose value like a vehicle does, usually grow in value the older they get.
If you are set on selling it there is a lot of variables to what it is worth. Condition of tin, engine, tires, how well it starts and runs, any factory options will all play a part in the price.
You can probably get $1500-2500 for it, mind you there is one for sale right now on tractor house for $15,000.
Posting pictures of the tractor will help us put a more accurate value on it.