This is one of those cases where the original window seal is your best option. As I said before, if I find something wrong with anything I sell I won't use it anymore.
In the case of the windshield seal, I found that the repro was about 2'' too long. You can get away with door seals and trunck seals but the windshield is a very exact fit so original is the right part here.
After going back over the posts I tend to agree with Matt C. that there is a difference between some parts. OEM is OEM but even here we find that there can be differences in quality. This isn't a mistake but rather is a price threshold objective. VW is a good example where parts made in Germany will be better quality and more exspensive than those made in Mexico. The SA parts are destined for places where people just want something that runs and quality isn't a big issue. A typical VW shop in the States wouldn't dare use these parts.
There are cheap motor mounts being made in China and other places and they will last a few months. This is in fact, true. However, there are logos on parts or their boxes that should give you a clue. OEM parts come in boxes that say, Jurid, Fibi, Bilstien, Sachs, Boge, Behr, Bosch, Lempforder, ect. MB uses these same parts.
You should look at a supplier that's been around for at least 15-20 years so you know they aren't fly by night operations that won't be there for you. These types of dealers will be handling OEM parts and have a reputation of seving the comunity long before ebay came along.
Having said all of this I don't agree that your only reliable scource is MB. That's not true. If the price is really low on the same part you can almost bet it's a cheap knock off. You want a bargan, put up with the problems.....