I would say if the $5000 the OP has in mind is some over what is owed, that would be a fair offer to start talking price.
This sale doesn’t seem to be about getting all you can out of a nice horse.
Money is always important, but maybe takes second seat here to getting the horse to stay there and with a congenial client that has proven to do things right.
Last client has been defaulting for a while now, barn owner not being paid, why horse is for sale now.
That is worth accepting a lower but still fair price between parties that are looking after the horse’s well being more than what all they can sell him for, or the buyer trying to get a good deal on a horse, when it seems that she likes the horse is first.
If sellers want top money for that horse, they would have to put the time and effort and money to carry him longer, get him fit and showing and all going well, maybe then have a horse of a higher value.
They don’t seem to want to go there.
As others have mentioned, age of the horse, mid later teens, means his use needs to be managed to reflect that and he may go on like the energizer bunny for many more years, or be hitting old age soon.
That would lower his value as he is, unless he was showing and winning right now, showing he still can beat others and hold up well.
Will be interesting how this situation goes on, maybe the OP and horse will be both lucky. 