Quoted from pinmister:

I would think that it would be hard to let go of a game where someone put their blood, sweat and tears into it. The machine almost becomes your child. It seems like guys that do restoration work are ok letting go of machines as long as another project is lined up.
I have found almost any game is replaceable, a few rare games may be the exception. One thing I have done in the past to help with the emotional aspect is to sell the game 'conditional'. Offer to sell-only if the buyer agrees to offer it back to the seller if they ever sell it(first right of refusal). This insurance helps with the emotional loss by reassuring the game could possibly come back at some time? I have had this offered to me multiple times now, and fortunately by the amount of time that passed I no longer had the desire for the machine(been there-done that). If anyone has difficulty letting go of games like myself-may try this approach.