It is prohibited to use these services in their ToS, and so how can I avoid being found selling btc with them?
So you're the one selling BTC? Isn't it risky for you as you are the one making an irreversible payment? Do you have any successful deal in the past? What payment gateway you used?
As for me, if you really want to avoid their terms, then refrain using them and start thinking of an alternative. Maybe used a crypto exchange with fiat deposit feature or just stick with your regular customers so you will be sure that somehow no reports of chargebacks against you. Meetups would be nice but only for small amounts and just around your local.
Only things I can think of is telling your buyer not to send additional message with transfer and do as few trades as you can on account
It will work but still, it can't guarantee you that it won't give you a problem someday.