The bag is purely for advertising Ebay. It is the label you pay for to use with a bag of that size and weight limits. C & S satchels are "generic" so that the Ebay advertising is not used on items that are not Ebay origin. In short  C & S are aiming to go mainstream not just Ebay.

Ebay bags are temporarily allowed on C & S, buit will stop as Ebay are no longer paying  C & S for advertising space.