There are exceptions jkazzoun, and i am also one of them. To play both sides of the fence....
As someone who refuses to go to the dealer (and hasn't), I will continue to buy my parts online as I have for a couple years, and take them to my mechanic to put in. He's a one man shop, and used to work for the local MB dealer. He also works on non-german cars so he also works on my passat and other family members cars. Does he complain that I bring my own parts? Nope, as long as they are good quality oem parts(which he knows I always bring).
Sure, I myself know that he makes a tiny bit of money on parts if I was to buy them from him, but it's not enough to lose on the current labor costs that I have spent with him, and as a solid repeat customer beats ANY new person walking in the door.
He also now refuses to advertise and only really takes in referrals. You got new people playing you and calling asking you to do "circuit city" type price matches. Most are tired of playing that game. He get's solid repeats like me and risks not making as much. Like black mercedes said, no one wants to deal with people spending hours trying to save $5. The dam dealer thinks if you buy a benz, then you shouldn't be cheap and try to bring your own parts in. Pay the dam $90 and hour, inflated parts prices, and just deal with it! "You could afford the fancy car to begin with, why you being cheap?" Well there is an alternative and that's the way it goes.
And for the dealer, yes, they do just fine for the specific reason that they know most people won't do what some of us do and that is why both local benz dealers in seattle make enormous amounts of money. And now that I have a benz, I know growing up the folklores that I would hear of benzes costing tremendous amounts of money to maintain and fix. Well my car's parts and labor cost me less to maintain and fix than my ES300, G20, Legend, and even my ford probe. It is possible
Now as for the dealer, I can only agree on one point. When I used to work for car stereo shops, we outright REFUSED to install parts not bought at our shop(warranty schmorranty). This was at a local one person shop and a retail chain. No way, no how. Who knows where and what the parts have been through. I personally saw bad things happen with "brought in parts". so as for the local mechanics on this forum, I can feel your pain and for this reason solely, respect your policies and feelings on people who want to buy their own parts and bring them in. But don't think people are out to get you and reduce anyones pay. If I knew that the mechanics I was staying away from were starving and couldn't put food on the table(because of a few of us), then I would think a little differently and possibly feel guilty. Sure they don't see as much as the shop does, but me myself is not putting a big dent in your paycheck.. There is so many people who don't have a personal mechanic that works on benzes, and have internet knowledge to buy parts, that the dealer shops are just fine with the little lady who bought her 500SL and takes it into the dealer and pays their prices. This goes on and on and on.....
Business is a dog eat dog world, and if there was no competition in this sector, imagine EVERYONE having to pay $90-100 hr labor, plus full retail for parts. Scary thought. Then the DOJ would have to investigate :->