"3.1 Sharing Account Information. You may not share your account or login information (such as your Square Enix ID and password) with anyone except your legal guardian, your minor child, or Square Enix. You are responsible for maintaining the confidentiality of your account and login information, even from your friends and family. If you violate this provision, you may unexpectedly lose access to your account to a third party without recourse against Square Enix. Square Enix will not be obliged to credit your Square Enix Account with Crysta or other points or awards, which may have been used by a third party as a result of the violation of this clause."
This is merely SE covering themselves in the event you sharing account information leads to unauthorized use. Basically, you bare all the consequences if someone screws with the account you gave them access to.

As for the advertisements. None are specifically against the ToS since they don't mention account information in-game. Whether it promotes RMT or not is irrelevant, and has nothing to do with the sellers. Plenty of people who buy clears have millions worth of gil all from legitimate means, especially with how easy and limited in value it is. Regardless, should SE ever attempt to ban sell advertisements, they'll simply be worded different: "Want to clear Omega? Message on discord to discuss setting something up."