Hi searchpl,
If you are worried about "freshness" for ecommerce sites, there is one very important thing to do - eliminate wasted effort.
The fact is that what I call the "freshness effect" does not apply to every keyword term. Google appears to be determining whether fresh information is, or is not more relevant according to the individual term. If you manually check SERPs you will see this easily.
So, eliminating wasted effort while working toward providing new and relevant content all comes back to good old fashioned research. The smart approach is to spend some time manually checking SERPs for your "money" keywords. If you see evidence of the "freshness effect" for particular terms, those are the ones you could consider focusing new content development efforts on.
The keyword terms that might be affected will entirely depend upon the types of products in your stores - for example, I know that "weight loss" is a term where the "freshness effect" seems evident in SERPs.
Of course, if you decide to develop new content you should follow the advice already given by EGOL and James on quality and method. Incidentally, I would say the smart thing to do in this situation would be to come up with the type of content that is easy to add on a continuous basis - things like ongoing series, videos, podcasts, and cleverly managed user generated content.
Incidentally, if you listen carefully to information coming out of Bing via Duane Forrester, you may notice that Google is not the only Search engine that takes notice of freshness 
Hope that helps,
Sha
Also: Don't stress too much if you are using automated feeds to update your product offerings on a daily basis ... you may already be providing fresh content if products are frequently added. The challenge then is to ensure that quality is up to scratch 