Well, I'd say the proper way to deal with them is to sell them to cover labor and parts for the smith, just like anything else. As to when, hopefully it would be after a period of time agreed upon before the work was done (preferably in writing). The only special consideration is that obviously thorough documentation should be kept (and forwarded to the customer) in states where registration is required. And obviously I'd say it's common courtesy to make a good faith effort to contact the customer before you sell it.
But that's just me. Haven't had any worked on, so I have no idea how it actually works.