Caporegime
Don't know how feasible this would be, but it might be good if you could allow MM access for people who have bought from OcUK in the past.
Presumably those people then have an account on the main site with a real name and address which could be tied back to their forum profile for trading purposes.
I know there is already some sort of linkage to allow forum posters their free postage.
It would encourage people to purchase for the shop if they haven't already done so and ensure that MM is populated by people who have contributed at least a little to OcUK.
But that still wont stop what has recently happened. Just making it more difficult to get access is the best way to help weed out the scammers who will just find it easier to scam elsewhere.