I don’t know that anyone would buy a 5 year old Jack Russel for any price. The only two situations where someone might want him is to make him a show dog or a breeder. In both cases you would need his ACK certified papers and prove that he is purebred (along with all his family history to inflate the price).
If you did sell to someone who was using him for these purposes, he probably wouldn’t have as good a life as he does with you.