I feel that it is outright scamming, you aren't supposed to sell accounts and the related trust. While the selling of accounts is allowed, though I think most people in the community frown upon it or don't condone it, but nevertheless it is tolerated by the mods and admins.
It's really only allowed so that users aren't left with the false assumption that it doesn't occur. It's simply impractical to avoid. By making it openly acceptable, there should be no false assumptions.
Obviously IP address account limiting doesn't work (VPN/NAT etc.). How can a website ensure that a user doesn't create more than one account without collecting/verifying personal information? Mike Hearn in a talk a few months ago brought up using a hash of a passport to uniquely identify users and limit them to one account. Not quite sure where that went but it seems fairly unworkable anyway.