The OP makes a valid point though. I don't understand why accounts are allowed to be openly sold on here
I don't speak for Theymos, but this question has been asked enough that I have the answer pretty well memorized.
Account selling is allowed for the same reason having multiple accounts is allowed. Because theres absolutely no way that we can 100% enforce account selling and multi account use. By making it openly known that it is allowed, people can stay vigilant. If we decided to crack down on account selling and multi account users, people would be lulled into a false of security, and those that slipped through the cracks could do more damage.
As it is now, everyone is aware that anyone could be a purchased account, so people tend to look for things like, old accounts that haven't been active in a while, and differing behavior than normal.