You might have a shot at selling it on M.net - those guys like to keep it stock. In nice shape (which it appears to be), maybe $40-50? Aftermarket units can be had for not much more with better everything, so it's really only desirable for someone that wants a nice OEM unit.
I was/am interested in the radio, but have a question. Does this HU requires a separate amp or other alteration in order to work in a Miata that didn't have this HU originally installed? In other words, is it plug and play?