On-line OEM part numbers source???

[donbryce]

Does anyone know of a link to enable looking up the original Mercedes part number for something? For example, I have a line on an EBay item that the seller has given a MB part number, but there is no where I can look this up to verify it is for my car.
Conversely, I've never seen an aftermarket web supplier that shows the MB part number, only their internal stock/part number.
Yes, I know that I can go out and try to read the number off the part itself, but this is often awkward (eg., a door seal rubber. The car is in storage right now, and even if it was in the driveway, the number is hidden).