Is it even allowed to sell your bitcointalk account? and why would you want to sell it. Being Legendary gives you loads of perks.
It is "allowed", but it is heavily frowned upon. Buying and selling Bitcointalk accounts encourage spam, scamming, and shit posts. You are free to buy and sell accounts, but there are quite a few of us in Default Trust that will leave a negative rating on any account that we can see has been bought or sold. If that happens, you won't be able to enter any signature campaigns and the account will be close to worthless to make money with.
Buy accounts at your own risk.