First, you are going to need https://letmegooglethat.com/?q=Steam+Inventory+Helper+extension+for+Google+Chrome "Steam Inventory Helper extension for Google Chrome" that I can't link here, but you can ask Google.

When you have installed it and restarted your browser, then log into Steam with it and go to your inventory where you have cards to sell, then make sure to select these options for cards:
and if necessary "Select All" on each page.

To sell them click the RED button that says "Sell XX items", click on "Auto-sell", check that you accept terms and set the price automatically, then click "Ok, put it on sale" and confirm all sales on the steam application by phone, now just wait for your steam wallet to be filled.

You can Follow our variety stream on twitch/ThrilledGamersTV 👍 as well as the Steam Curator page 💖 in English 🇬🇧 (and the Italian 🇮🇹 version) to get our advices on the Steam store.