1. Blog on a subdomain.
Though sometimes easier for administration, subdomains might dilute your SEO efforts. Links to a subdomain don't count 100% as links to your main domain. A /blog/ could help with the generation of incoming links and addition of fresh content to the domain you want indexed a lot.
2. Employ Canonical
www. redirects to non-www. Trailing slashes get added automatically. So far so good. But it is still easy to create duplicate URL's by adding random dynamic variables.
Without Canonical an URL like /boats/?dupe=content will point to the same resource as /boats/. Here you might introduce a canonical problem. (http://googlewebmastercentral.blogspot.com/2009/02/specify-y...)
3. Optimize your site for speed
Though not that many search queries are affected by the site-speed algo, site speed remains very important for your visitors, and so indirectly for your SEO/marketing efforts. Google Site Speed plugin, Yslow or these guidelines (http://developer.yahoo.com/performance/rules.html) might help you fix some of these issues and gain a few seconds.
Mostly loading javascript just before </body>, turning on caching, and compressing and combining resources.
4. Robots.txt vs meta robots
/search is disallowed in robots.txt. If you disallow it on a page basis, with meta robots, you can specify: "noindex, follow". That way if people link to your search results, link juice will keep flowing through your site.
5. Breadcrumbs
Add rich snippets mark-up. For product information and reviews, but an obvious contender is the breadcrumb. Link to your twitter (and future Google+ profile) with 'rel=me' to signify ownership of your graph.
6. Images
Add an alt-attribute to the site logo. Specify the dimensions for faster rendering.
7. Don't critique ehow.com if you fill Google's index with 245.000+ automated results.
Or put less bluntly: Write more unique content to introduce bigger categories. Add more relevant content to your listings (reviews, search/trend data, price watch).
8. Make clear if an item is "already sold".
If I click on 10 entries and I get 10 times "item already sold" I start to doubt the usefulness of the application. I compare this to a job site, where the jobs are mostly filled: You happen upon such a site through Google, because Google still thinks these listings are relevant.
9. Quality
The site is mostly void of trust factors. Due to some listings being in ALL-CAPS, some result pages can look a bit spammy. Add more trust factors, and try to repair spammy listings.