This was a great question/blog - I've often wondered the same thing about these eBay "stores" and who would buy anything from them when they are easily 2x or more over reasonable fair market value. I hadn't seen yepbg's stuff until recently - the ones that always stood out to me (aside from the obvious in Levi/707) is Higbee's and the examples shown earlier from bighitsportscards2008 (as in you take the "big hit" if you buy anything from them)
What gets to me more about the eBay model is that whenever I do make an offer to someone in Stores, I'm usually countered with an "off eBay" response - yet eBay gives many these sellers their "Top Seller" certificate/stamp of approval!
