No in my opinion I don't think it is. Furthermore, there are no laws that ban prohibit the sale of any social media accounts whatsoever. As long as the account wasn't stolen or hacked and there was a formal agreement between the two parties(buyer and seller) then I don't see any reason why it should be illegal.

What might be illegal is impersonating the person that sold the account to you though. People tend to use ploys like that to scam people into doing rather unscrupulous things so if you actually want to sell your Instagram account then be sure to know why and what the person that's buying it is buying it for.