UA can largely prevent this by limiting the number of people in a family account, by limiting the reasons for and the frequency at which members in the family can be added/removed. It could even make redemptions using transferred points only available to the lead member of the family account and the members who have contributed points to the family account. Last but not least, flyers with "tons of miles" obviously have more at stake if their accounts are shut down as a result of them engaging in selling their miles.