To post a link (using a Windows PC) go to the relevant eBay page, right-click in your browser's address bar (which will highlight the address) and select 'Copy'. Come to an AB post, right-click and select 'Paste'. (Make sure that you don't type anything immediately in front of, or after, the link as it will get mixed up in the link and corrupt it. It's best to put your link on a new line and then press 'Enter', so that the rest of your post is kept separate from it).
To do the same on an iPad, click in the address bar (when viewing the page you want to link to) and then on 'Copy'. Come to an AB post, press and hold where you want your link to appear (preferably on a new line). When the magnifying circle appears, stop pressing. Then accept the 'Paste' option which will appear.
[Note that your link will look just like normal text on your screen. It will only become a red, clickable link after you've submitted your post]