Even for the well educated, shopping online and simultaneously being an informed consumer can prove quite difficult. Recently while surveying a Mercedes part's website for air mass meters, I was presented with three different choices. I assumed one was rebuilt (not a great idea), one was OE (since it was more expensive), and I believed the third to be an aftermarket part. Bosch of course is the OE manufacturer Air Mass Meters (AMM), and over the years I have bought them in factory boxes, Bosch Boxes, and by purchasing the inserts. I have also purchased aftermarket AMM with limited success, so buyers beware if you are so inclined...
Recently I received a sales catalogue from the Far East with different electronic components, one of which was an air mass insert. My cost--$9.00! The point is, to get a quality product you must trust the vendor, you must be somewhat informed, and you must realize that there is a vast difference between quality and price among parts. Obviously a part in a Mercedes box is the best, but oftentimes the same quality can be realized from the OE manufacturer. Today most parts are being manufactured where costs are lower for labor; China (of course), Mexico (large percentage of ML parts), Turkey, Brazil, India, along with others. We see a lot of suspension components from China and the quality will never be as good as the OE, even though the OE is manufactured to a lesser degree in the Far East. Among Mercedes parts you generally want to look for manufacturers such as: Bosch, Behr, Bilstein, Lemforder, Siemens, Ate, Girling, Zimmerman, Jurid, Pagid, Hella, and Beru. There are others, but these are the ones that come to mind at the moment. SO--what can you take away from this discourse? In short: price is not the main factor in buying your parts. Trade with someone who is reliable, service oriented, and has knowledge of the parts. Make sure you provide your VIN # and deal with a reliable vendor, preferably one you can talk to in person.