A thread at SEO Chat Forums named Optimizing eBay Store asks some questions about how to rank one's eBay store well. SEO Chat Moderator, EGOL, gives advice about the standard eBay store;
If you have the standard ebay store you will get five pages on their domain plus an "about me" page. The more items you place in your store the higher up in the store index listings you will be in each category. This will pass some of ebay's PR down to your store. These pages will show in the search engines. To optimize the pages in your store you need to know a bit about SEO and be pretty ingenious to "trick" their system to put your optimization in place. You control the second half of the title tag plus onpage design.
If you really want to have full flexibility, then I recommend you check out the eBay Developer Platform.