Veek,
Yours is a very good question. I have a friend who has over 100 cars, including both a Safir and genuine GT40, and his wife has zero interest in them, and matching knowledge about them. He hasn't yet stepped up to the task, but he wants to build a complete portfolio on each of his cars, detailing their history, purchase price, perceived value at the time the portfolio is created, etc. so that if his wife should ever find herself with the task of disposing of them, she would have something to go by.
I would think that an SPF dealer would be a good ally in your case, as they would be better positioned to market the car to the appropriate target audience than XYZ Used Car Lot would be. So I say pick your favorite dealer, and have an arrangement in-hand ahead of time so that if the car has to be sold in your absence (for whatever reason), they are positioned to handle the transaction on your wife's behalf.