i would search for the statement but yeah they don't support it beside not being ilegal (u can read all the rules and there aren't rules for selling items as long as u dont advert using irl money)
Rule 2c"Advertising trades for real money:
It is not allowed to advertise trades of any kind that involve the paying of real money. This does not only apply to the trading of real items for cash, but also to offers to buy or sell Tibia items for real money. Note that it is also illegal to advertise game code trades as game codes represent real money."
I think Tibia Coins represent real money so is just a loophole that they made on their rules they don't support this behaviour but they wont punish you unless u advert it, so is totally under your own risk.