How to buy and sell cards
In spite of the name of this site, you can do many more things than trading cards in it. One of them is buying and selling cards to and from other users.
This is very simple, it just means exchanging cards for money instead of exchanging them for other cards. So you can use the Tracked Trades system normally by entering items to be sent by one of the users and none by the other, and adding a cash exchange where the latter sends money to the former.
For example, if you want to sell your cards, you can send a Tracked Trade offer to another user who wants the cards you have and indicate that the other party will be sending money for the price you want for those cards. If the Trade gets accepted, the process is the same as with a normal one, only that, when it's the buyer's turn to send, they should transfer the money to you, ideally The buyer needs to make sure they have enough funds in their account for this purpose. You can learn about money accounts here.