Like I said before, charge 10 or 20 bitcoins for a user account but that money is put in said account. it can be used to purchase items. it cannot be transferred out into another account. that way, they would be forced to do multiple purchases from one account. and making multiple accounts would be a pain. plus, they would get all their money back to purchase items... Also, make it so that a new user can only do one transaction at a time til he reaches 2 or 3 successful transactions.. Im just spitballing. giving some ideas...