Well, the first thing I would want to know is if it works. Does it sew smoothly and reliably? If it does, sew up a few small items and leave them on the machine as samples, maybe a pretty cloth napkin and a mug rug. I'd also list or show whatever accessories come with it. If there is no owner's manual, I'd download one and put it in a clear report cover. Everyone wants one of those.
Before the benefit, I'd clean and oil the insides as well as clean up the outside and the desk. Take care of those decals which are in pretty good shape. Craigslist and Ebay completed auctions will give you a general idea of asking and selling prices, but it's universally true that it is worth exactly what one person is willing to pay for it.