Is it from a dealer? Does it have warranty? Did you look at it and know what shape it is in, or is this something someone told you? Has it been stored inside or outside? Is the loader quick connect or pin-on bucket?
As you can tell, there are lots of questions not answered in your post. I'd start at $17k as a median and go up and down depending on the positives and negatives of the tractor. I think $18.5k would be my top offer in the first round of negotiations, and I would not expect it to ever sell below $16k unless the seller is really stressed. If it was a pristine tractor at a dealership and had warranty left, I might go as high as $19k. It would depend on my needs and the fact that this is prime tractor selling season.