I think if you have a very late model tractor with low hours, like you do, I would start my calculations like this: A customer at a dealership wants to save 20-25% off of new if he buys a late model low hour used tractor. Anything higher than that and most folks can justify buying new for reasons of warranty, financing, new car smell, etc. Not 20-25% off MSRP, but 20-25% off a normal selling price. If you take it to a dealer to sell for you, he may well ask 15% commission. So lets say your tractor was $50k new, knock off 25% and we are at a selling price of $37,500. If you decided to have a dealer sell it and the dealer then got 15% you would net $31,875. So using these numbers, you would try for $37,500 and think seriously about offers that exceed $32,500. Of course if you really aren't in a hurry and have no immediate need for the money, you might set the lower threshold up a bit.
On a higher dollar rig like this, the dealer may lower his percentage. 15% of $37,500 is more than he made on it new. 15% on an old 8N Ford selling at $1800 isn't worth the time for a dealer to sell it, but as the $$ go up, things change.
BTW, I just used $50k as a theoretical price, I suspect that might be a little low.
I think Navasot and I are on roughly the same page, I just get paid by the word....