AZ and some of the other reboxers do sell a lot of parts and they command some great power purchases from companies like Bosch.
As an authorized Bosch Service Center, I can tell you that Bosch has three methods of distribution: OE, their warehouse jobber system, and the large store accounts. The large store accounts usually only buy large quantities of a small number of items. They will probably not be able to order any item not on the store account list except through the second distribution line (or the first).
The product they sell is the same Bosch that comes from any other source, but will usually only be rotating electrical (starters and alternators) and service parts: filters, plugs, etc.
As to plug wires, Bosch isn't OE and MB's way of dealing with wires requires real technicians because they would only have you fix what is broke. Since real techs are few and far between, they now sell Bosch wire kits at the dealers. I am pretty sure there are a couple different quality of Bosch kits, some even come with the quality Beru pieces.
Bosch Master, ASE Master, L1
33 years MB technician