When you are typing or editing the description there are two "tabs" at the top. You are using the first tab to enter text - the second tab changes the mode to "HTML" and you can now enter a link. The easiest way to do it is to use a copy and paste just like you do to insert the link to your eBay auctions at the bottom of the TCB page. Just go to the page you want to refer to and copy the address from the address bar at the top and paste it in the description while the HTML tad is active. EBay have a somewhat restrictive policy on what links are allowed - here are their words

Though most links aren't allowed in listings, you can include links to:

Other eBay listings, Stores, or pages
An email address members can use to ask questions
Descriptions, photos, or terms and conditions for your listing
Websites that provide a service for your listing

If you include a link that's not allowed, your listing may be removed, and your buying and selling privileges could be restricted