Electric or handcrank? If electric, do you have the dome case and the knee bar? Knee bars seem to get lost a lot (they fit in a clamp at the top of the case), and the machine isn't really useful without it. Is it clean? How are the decals? These are the big questions. An electric machine with scratched up decals, bad/dangerous wiring, and no knee bar will be worth at most $25 in my area since it is only good as decoration. A nice clean machine with good decals, handcrank or with good wiring and a knee bar, in a dome case could go for $100 as Charlee said.
Don't be fooled by the eBay ads saying a 99 is worth $300; they really aren't.
Pam