It is not illegal but is grounds for termination and suspension of the account, if proven guilty.

One subsection in their Terms of Use  is this:

Source: https://help.instagram.com/581066165581870/?helpref=hc_fnav&bc[0]=Instagram%20Help&bc[1]=Privacy%20and%20Safety%20Center

Depending on how the account is used, most likely for fraud or public deception, then at that point will one's action be illegal. But as to the matter of selling, the only thing that Instagram can do is to remove the content, suspension and/or termination of the account in question.