I bought my 2000 E320 used without getting the user manual. Currently the maintenane B lights on and MB warranty has already expired and I wonder 1) what exactly is maintenance B and 2) can a regular shop reset the light after doing the required job.

If you have lost or misplaced your Mercedes-Benz Owners Manual and you would like to order a new one, simply contact a Mercedes-Benz Representative and they'll be happy to help you. Please do not hesitate to:
Call us at 1-800-FOR-MERCEDES (1-800-367-6372)

You can also check out http://www.mbusa.com/brand/index.jsp for information on B maintenance. The insturctions for resetting your light are in the owners manual. Yes a regular shop (or even you) can reset the light before or after doing the required job.