Tibia Coins can only be sold in the Market, gifted to other characters or used to purchase casks in the Store if they are transferable.
Tibia Coins which are not transferable can only be used to purchase things for your own account (except for casks) in the Store.
When you purchase Tibia Coins in the Store, you can select whether you like to buy transferable Tibia Coins or Tibia Coins which are initially non-transferable (note that they become transferable 6 months after the purchase). If you are not allowed to purchase transferable Tibia Coins with the selected parameters, you will have the options to either purchase Tibia Coins which are initially non-transferable, or to try a different payment method.
Trade Tibia Coins!