Will I get banned for buying coins?
No, the first offence for buying coins if you are detected is a coin wipe. As long as you spend your coins on cards, they will have nothing to take.
However, I do take certain precautions such as not using max price range, multiple accounts and multiple types of cards. This lowers the risk of being flagged by EA.
If you have already been flagged in the past, please buy coins at your own risk as the second offence is a club deletion.