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 our supported marketplaces here (such as TokenTrove) and trade with other players. 

*Note: Plain quality cards (cards with grey borders) aren't NFT cards.