From the FAQ (https://www.tcdb.com/FAQ.cfm?ITEM=General#5):
Are the cards listed for sale?
The Trading Card Database does not sell cards directly.
On pages for cards and sets, you will find an eBay section where items for sale are listed. These items are for sale through our eBay Partner Network and this site receives a commission.
Underneath each card is also a link that shows how many times the card is mentioned in members' collections. There you can find if the card is for sale or trade by a specific site member. You can then send the member a private message or make them an offer using the Transaction Manager.
There is also a Buy/Sell/Trade forum where you can find items for sale by site members or post about items you're interested in buying.
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 3/4/2024). I have added a large portion of my inventory to the site, and currently have trading turned on (details are in my profile).