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Why are some items "dealer items" ?
Car manufacturers make some items in their own facilities, and only sell them through their dealer network. Typically these are body and trim items, some emission and electronic items, some switches and knobs, and other items that vary from factory to factory. Some car companies operate a parts manufacturing subsidiary that sells items on the "aftermarket" as well as through the dealers.
Examples are AC Delco (now Delphi) and Motorcraft. There are other items that the factories buy from specialized component manufacturers on exclusive contracts that do not allow the component manufacturers to sell on their own.
Items that are usually available from independent sources ("aftermarket") are ones that the component manufacturer retains the rights to sell to customers other than the factories (one example is Robert Bosch electrical and fuel injection components), or repair items for which there is sufficient demand for an independent manufacturer to tool up and duplicate.