I would just make it a straight trade. I think since we are fully involved in "the scene" that we tend to forget that these cabinets, especially a fully working one, is something unheard of by a "layman".
Since there are a couple of problems that you stated with the buttons and coin door, inform them of it, and let them know you will fix that part at least, (let them worry about the cosmetics) and then the trade to me seems to be quite fair...