Theory: EBay as Flea Market
Let's assume EBay looks the way it does (not great) because not a lot of attention was paid to the design. Now let's say they had contracted the design to a professional services firm that practices user-centered design. What would the result look like? Most likely something pretty slick.
Conventional wisdom - at least with the folks I hang out with - says that auctions, plus EBay's first-mover advantage - is such a compelling experience that people will tolerate the bad design. But what if EBay is succeeding because of its bad design? What if, like a flea market's rough, seller-created environment, the amateur design communicates the idea of bargain?
A designer might have come to this conclusion - balancing some good global elements like navigation with lots of seller-created pages, letting the vernacular bubble up, however painful to look at - but maybe not. And even if the designer was to hit upon this idea, how hard it would be to sell, or even to think about selling, a poor looking design to the client.
The experiment to switch the quality of the design could certainly be run, and will be if EBay ever grabs the reigns in a big way and puts a pretty design in action. If popularity declined as a result, that would be quite a big insight into experience design.