I'd much prefer there was one place to go when you need to buy something. One directory where companies would place their announcements about their products services with full descriptions and current prices and cathegorize it in all ways possible. Directory would be diligently moderated and flaws of the system that could give some people unfair advantage would be fixed daily.

It almost already happened for used goods. If you want to buy used good you go to ebay. If you want to sell used good you adertise it on ebay.