get it under the table, sell it the same way.
Officially it seems it's close to impossible. A friend I met in Malaysia and again in Souther Sumatra had a good mate of his visit and bought a bike for him. Simply rock up in Medan, go to one of the cheaper hotels, if you're not already there, and say you want to buy a bike and than relax. TravFella has a real nice owner and also did the tricke for them.
They bought the bike there within 1 day, a used 180cc Hondacopy, dual-cylinder for 11.000.000. They sold it in Maninjau close to that again. Would have even got more, but apparently it's an issue to get the plates changed from Medan to Maninjau.
As his was a big-ish bike you'll get the scooters for less.