If you are buying and selling codes one-by-one and it’s not too time-consuming, you could embed each code into a text file and upload to an app like this one: https://apps.shopify.com/digital-downloads (I’ve used this and it’s pretty no-frills but it’s free, it works, and it’s developed by Shopify so it’s reputable.) If you have a ton of codes to enter and need a quick way to import them, then I’m not so sure. This one looks worth checking out but I know nothing about it: https://apps.shopify.com/foridev-selling-codes-app