That is against the TOS. Unfortunately it looks like the seller closed the account or changed the password. There is nothing we or EA can do about that.
To continue playing you will need to purchase the game on your own Origin account.
What @jpkarlsen said is the truth. The game is on sale right for like 1,5 more days on Origin I think. Buying it on Origin is the most secure way, but you can also check out other trusted sellers here.
If you choose to buy the game in the store, make sure that you buy a sealed box (with plastic around it) and keep the receipt.
This was hard for you to know as a kid, and very immoral of that adult who sold the account to trick you, I am actually a bit angry. Make sure to talk with your parents when spending your money so they can help you make sure you're buying the real product.
Maybe you can contact the account seller and confront them if you want to.
Good luck
