Ecom incorporates a lot of onsite elements. since ecom's are usually larger in scope, they tyend to have more negative quality factors and duplicate content.
Products descriptions are usually just copied from the manufaturer. I was actually involved in a large ecom redesign for a large corp. I demanded we hire writers in house to rewrite all product descriptions. Not everyone can do that. We had a high $xxx,xxx budget for the site build.
Canonical URLs, ftw. Dont let things like session ids or products in multiple categories or whatever bring you down. Select a preferred URL and go for it.
PAy attention to your site structure. Categories need to be logical. I like with MAgento how the product can exist on site.com/product.html, no matter what category it is in.
I took a brand new site from start to finish and it started ranking within a couple of weeks with no offsite SEO. The structure, brand and socail signals send quality and trust signals to google. The site did $25,000 in revenue organically before we ever built a link to it.
Set it up correctly first! Then thing about off site strategies. Anything you do to improve the site quality will make your off site efforts much more effective.