Go to my account, affiliate settings, click Ebay, then edit...There will be a link in the help section for you to sign up with..
HOWEVER: Please note the following change in Ebay's TOS
eBay Affiliate ID Help
"Note: eBay has changed the terms of their affiliate program, and is not currently approving applications of people that do not operate a site on their own domain. If you have an approved ePN account you can use it on HubPages. If you do not have an ePN account, you are very unlikely to be approved by applying using your presence on HubPages. If you have a site on a domain that you own, we'd suggest applying using that site.
You'll have to sign up for the eBay Partner Network at affiliates.ebay.com. They'll ask you to fill out a series of forms with your name, address, and payment information, and to describe your site. It should just take a few minutes, especially if you follow these steps (will launch in a new tab/window). In order to be approved it's very important that you include accurate contact information, and explain in some detail how you will promote eBay products in your hubs.
After you are signed up, you need to enter a numeric eBay Affiliates Campaign Id (usually a 10 digit number) in the space above. You can create a special campaign just for HubPages, or use the default campaign that comes with your account. Look under the campaigns tab after you've signed in to your eBay Partner Network account."
Hope this helps.