My local Mercedes dealer sold me a used 2008 C300 that, as it turned out, had the wrong size tires on both front and back. When I found out about this I was upset, but they will not do anything about it.

I am getting the tires replaced, but am wondering what brand of tires the 2008 C300s were originally equipped with?

 

Go to tirerack.com and follow the directions. As a sample only, OEM tires for the 08 C300 'luxury sedan' were Continental Contiprocontact 225/45R17 or Michelin Pilot HX MXM4 225/45R17.
There are other different models listed for the C300 i.e. 4Matic, Sport, 17" and 18" etc so check for yourself. Once the listing appears you can also ask for OEM replacements.
I have found the accuracy to be excellent.