First, what does the same model tractor and shredder sell for brand new?
Next, how much did you put down and how much have you paid to date?
Finally, what are the headaches cost to you?
I would offer that what you put down and 24 mo's of payments exceeds $5K.. Therefore, if your total OOP payments from day 1 are $12,000, then 12K - 5K = 7K/24 mo's of operation, you basically leased the tractor for $291/mo
If you want more than the $5K they offered, then do the math on what it's costing you/mo, how long to sell, all the transaction costs/headache costs, then you anticipated sale price and you have your answer.
To me, if someone would have said I'll give you $5000 for it now and you can walk away, you would have taken it in a heartbeat. But, the thought of making more has stopped you. Take their offer and be done w it as you stated in your opening statement...good luck