Demand for the M6 on the used market is strong and with such limited supply, private sellers are likely to take a chance and could be asking silly money. Buying a used approved one through a BMW dealer might actually be the most sensible option - if you can find one. The M6 could be seen as better value than a used M5 as the £20,000 premium shrinks rapidly on the second hand market.
If you're selling an M6 then you're in a strong position and don't be afraid to ask top price. With such a limited supply you won't have to wait very long for a buyer.