A lot of people have had that same complaint. eBay is secretive about their criteria.
In addition to good content and design, your site should also have:
- Contact details
- Terms of service
- A WHOIS record that shows U.S. based ownership -- by you
And then, when eBay sends the first rejection notice, respond with a nicely written appeal. Apparently the appeals are working. That feels so strange, compared to Google.