As far as the yellowing, that's actually due to the flame-resistant compounds used in the plastic. Pretty much anything plastic and beige is going to have it happen at some point, some just faster than others. There are methods to reverse it, but it takes a bit of effort to do so.
Anyways, good luck with the sale! Seems like a great deal, I'm actually in the market for a Duo right now, but want something with region mod/RGB, been keeping an eye for doujindance's stuff, but all he has is a supergrafx right now :/