Your first sentence is spot on. What I think is getting lost in this conversation is that it makes more sense for retailers to incorporate Facebook-enabled functionality on their own website, rather than have an e-commerce store within Facebook.

I founded a company called Snapline (http://www.getsnapline.com) that helps retailers integrate Facebook functionality into their e-commerce site. We've talked to many retailers, and most are still not sure where Facebook comes into play for their business. Small retailers want to engage and increase their Facebook audience, but it's time-consuming to make consistent feed updates and it's impact on sales is pretty murky. Large retailers seeks to gain much more by engaging their Facebook audience, but implementing features on their site that leverage Facebook data is outside their core area of expertise. I think what can go a long way in solving both problems is incorporating functionality within an e-commerce store that doesn't require a lot of manpower and improves the shopping experience.