I am planning to buy 2012 e350 4matic. it has 13000 miles on it. It is not from Mercedes dealer. What kind of issues I should be checking / looking out for?

Insist on a CarFax report. It will show if the car has been serviced regular at a MB dealer and also any reported accident damage. That car is on or recently came off warranty so there should be good MB service records. Many dealers use AutoCheck instead of CarFax. That is because it is cheaper and does not show service records (or lack of them). Insist on CarFax. If the dealer refuses then go on to another dealer. A truly honest dealer will show you all that is good (and bad) about their car.

AND..... ensure the Maintenance Book is complete and filled out. It should corroborate the CARFAX. Also, run the VIN through an online free VIN decoder, to be sure you know what the car was born with. Finally, why was the car not traded in at an MB dealer? Seems like an ideal candidate for a CPO program.... just curious.... More than likely, you should be good to go with that info before you talk $$$ I found (IMHO) that the pricing on Car Gurus is useful and pretty accurate. Don't forget, it is 4 years old, so a large amount of the depreciation is in the fourth and fifth year. Good shopping !