It's a Duster front end, with a Cat series 400 frame, (maybe a Dominator, with that sissy bar and suspended seat, and plastic tank mount) with a scrub brake added. That is not a Duster scrub brake, as they were solid in the middle for the mount and on the other side, and no other 400 series frames had the scrub brake.
The front end, (handle bars at least) and rear shocks, seat mount and sissy bar are worth something to a person trying to restore one of the above mentioned bikes. Something tells me the seller thinks he has more than what he actually has. A Hustler would bring some money, but this aint that.