Instead of purchasing from third parties, you can accumulate millions of coins through in-game activities:

: Excessive coin buying inflates item prices on the Auction House, making it harder for other players to afford cards legitimately. How to Earn Coins Legitimately

: EA may remove all coins from your account if they detect illegitimate transfers.

: Third-party sellers may use phishing techniques or require login credentials, putting your EA account information at risk of being stolen.