I agree with Jeff. I think waxes and polishes would be good. Along with the other stuff you already have. There also might be a demand for wheels, seems like everyone is always talking about them. How about Tire Rack?
formymercedes.com (they have extended discounts to SLK NW in the past)
yes, Tire Rack
With a MBCA membership dealerships extend a 10% discount. This is parts and labor and accessories. Anyone wanting to purchase a new OEM product might consider joining ($45) just to get the discount, if not for the very nice national magazine and local section newsletter...
Really the only others that I may be interested in would be adds/info on other members' businesses. If I have a need for something that another member sells, manufactures, etc., I would absolutely look to them before some stranger.