What? Isn't that some kind of agreement that should be signed before someone purchases the programmer so it's legally binding?
you dont need a signature, since there is a contract when someone buys the licence to use x mod.
If I buy a used gun with xmod on it you shouldn't be in the loop. someone paid you for the programmer and used it for it's intended purpose you can't make rules midstream. That's like saying that if you sell a gun with a Dye barrel on it you should send dye $110 since a new person is using the barrel.
Agreed he could word it differently. He means that you aren't allow to sell your licence, so if you sell your gun you keep your licence (regardless if you use it or not) and the new user must purchase a licence.
You can't track these things once they leave your sight, you can't charge someone money for something you don't own (once you sell something it's not yours anymore) unless you have a legally binding contract when you initially sold the programmer I don't see where you have a leg to stand on here.
Arguably the license can be sold, but since xmod is subject to copy right you can't distribute any copies above the number of copies you are licenced to have. He does have a leg to stand on since he could sue for breach of copyright rather than breach of contract (as you suggest to sign a contract) however it is subject to having to sue to claim damages and criminally it wont be enforcible unless someone is distributing for profit...