Thanks for the clarification.
I don't know what advice your vet gave you when you had the horse nerved, but in my mind there are three possible outcomes from nerving 1.) you remove the source of the horse's pain and extend his useful life many years 2.) the nerves eventually regrow in 2 - 3 years and he returns to his previous unsoundness or 3.) he develops painful neuromas as a result of the nerving and is completely unusable.
Most potential buyers aren't willing to risk 2 or 3; which is why leasing is a more practical option for an experienced show horse.
Also a concern is why a 13 year old horse needed to be nerved. That's young for the procedure, and seems to indicate that either his conformation or his career seriously impacted his soundness. IMHO, having to nerve a horse at that age means you should seriously consider a career change for him - showing less, showing in fewer divisions, retiring from showing and being a trail horse.
So, if someone who really wants his show experience, they may be more interested in a lease than a purchase.
Particularly in the current horse market, I think you'll find it extremely difficult to sell him.