The main thing is to use the 301 redirect code which indicates a permanent redirect so that eBay URLs are associated with the landing page content in search engine listings rather than your redirect URL. At least, this is how I understand it.

So in PHP where there is a redirect, use the code as follows:

And in the .htaccess file where an external link is being served for the URL, include R=301 in the attributes of a re-write rule that redirects say an image link.