If you want to follow up with people who are purchasing something from you via an eBay auction, you need to make sure that you're getting their permission to do so.
Provided you have done that, you can use the Import feature to add them to your list.
People Viewing "Auctions", "About Me" pages, or "eBay Stores":
As long as you have the ability to edit the HTML for your auction, About Me page, or eBay Store, you should be able to add an AWeber web form to those pages.
Note: Prior to putting a form on one of those pages, make sure that you are not violating any of eBay's Terms and Conditions in doing so.