My guess, and my opinion if I were an AB editor, would be that a "single commodity" category would be too narrow. Also the potential for Users going outside the "rules" of the site is too big a risk to take. Such a category would be likely to give rise to 'swops' or even offers for sale, which are adequately catered for on other types of site.
(No, I wont change my username to WetBlanket!)
NoGood Boyo I would agree with you to a point, however it would be helpful to find values of car, insurance etc on this site without having to pay a fee especially if they are older cars as other sites charge you for this info.