I guess, there is no law provided stating that selling bitcointalk account is legal or illegal. Plus, there is no agency monitoring whose using, selling or buying  a certain account. So, I guess, it is up to you weather you want to sell it or not. Each rank worth of course.  Because it takes time and effort before a certain account rank up so, it really matters.  Its price of course depends on the agreed amount of your buyer.  But, if I were you, do not sell your account.  It can give you great opportunity to earn.