Your credit is not affected by how much you have. Do not sign new cards. Do not use them. Unsigned cards are more valuable than signed cards. Mint cards are more valuable than those that show signs of wear.
Trade duplicates with other collectors.
Cards that have expired may still have active account numbers. If you are trading cards with a magnetic stripe, run a strong magnet over the stripe. This will erase all information encoded on the stripe and make the cards unreadable at retail. Because of mail orders an uncancelled account could still be used. Make sure your account is cancelled .