I have bought some but never sold any. On the official site there is a market link. Once you click the market link you will see the cards listed and their value. Click on the card you want to purchase and select Steem / SBD as a payment option. You will be using Steem Connect or Vessel to access your Steem / SBD to do the transfer. The payment is then removed from your Steemit Wallet and the card you purchased placed in your collection.
Alternatively you could start a post and offer them for sale and then do the transfers manually with the buyer. The common cards that you hold and that I listed are worth from 0.04 - 0.07 cents each atm. The legendary gold foils have been selling for up to $3000 USD.
Edit : check out the #steemmonsters tag there are lots of easy competition for free cards. Most just require a comment / upvote.