To trade cards on the Immutable X Marketplace, you’ll need to own a wallet, and have that wallet linked from Gods Unchained to Immutable X (see here for a big ol' guide). This is because ETH, the official currency of the Ethereum network, and NFT items need a secure address in order to be held and traded.
Once you have this and you own a tradable asset* (like an NFT or ETH) you can head to the Immutable X Marketplace and trade with other players.
*Note: Plain quality cards (cards with gray borders) aren't NFT cards.