What URL can users bookmark to get back to that item and tell their friends? What URL can search engines index to show that item in the SERPs?
I would have said that an e-commerce site should be implemented initially so that it works without any JavaScript at all. You click a category (an HTML anchor) that makes another request and the server returns a page with the items in that category. Your site is SEO'd and works for everyone. Your site is "pro".
You then want to make it more whizzy and implement AJAX as a progressive enhancement. If JavaScript is available and AJAX ready then assign behaviours that override the default action of the anchors that submit requests to the server. The requests are now submitted by JavaScript, but the underlying search engine friendly HTML is still the same. Your site looks "pro".
When developing the site in the beginning keep in mind that you'll want to implement AJAX on top later.