Why not just create a new account (over a VPN) then only post from TOR? Easy way to post anonymously.
Because of this
Having a permanent account gives an observer the opportunity to link information from multiple posts together to build a profile of the real user.
For example, in one post a user mentions being from the UK, on another he mentions being an avid player of an online video game, in another he reviews the bitcoin exchange he uses, in another he mentions his brothers name. Over time these little pieces of information can be used to build a profile of the actual user and possibly deanonymize them.