Google can sense when someone 'overoptimizes' a page. I can tell you as a fact that it happens. My biggest site got a -50 penalty for overoptimizing a keyword (I repeated it WAY too many times on the page) and suffered the consequences. Once I removed about 10-15 instances of the two-word keyword, the site started climbing again.
There are a few simple ways - 1) Make sure your categories are all well named 2) Footer, Headers, Inner Links 3) Have free blog sites like blogspot etc write articles pointing to your store 4) Try some social bookmarking 5) Hire someone to do a few directory submissions
If i was you i would buy a domain ,setup a site with unique content related to your shop products then display them products via an Affiliate link on to ebay. Havent got a clue whether thats against TOS but it might be worth looking it to it , that way you would be doing SEO for your domain and not trying to promote ebay.
Thanks for to the point 5simple ways , I like the suggestion Also add do some article submission (UNIQUE to TOP Dir's like Ezine) Cheers ~Merlin