If the machine is a piece of crap and you'll just get rid of it, get a used DMD (which you probably won't find). If it is a game you care in the least about, just buy a new one. You're going to pay $80+shipping for an unknown used DMD that has a limited (in most cases very limited)life span, or you can pay $130+ship for a brand spanking new DMD.
FYI, I have an ebay saved search for DMD's, and I never see used ones. The one or two I have seen sold for $80+, and were as-is condition.