Guys I need some advise. I recently bought a Certified Preowned C300 Automatic two days ago and I have this shaking issue. When I start the car in the morning, the car's rpm doesn't go above 1,000 rpm then it has shake like a diesel truck. Then when I drive the car and stop at the intersection, the RPM drops to 500 and it shakes again. I took it to the Mercedes dealer this morning they said it is normal. I know the start up in the morning shake could be normal, but not if the engine has warmed up already. The dealer said there are no codes, but they will check again on Monday. I am thinking now to return the car due to my disappointed and buy another brand.

I noticed when I wait at traffic light, idle is at 500 in gear, at 700 in neutral.
My C300 has no issue at all, very smooth all the time.

^ You mean your car's RPM drops to 500 on idle and it is still smooth no shaking at all? So 500 RPM is normal with these cars?

Yes. I test it with my left hand on the steering wheel when I am idling, and shift between R and N. I feel nearly no difference. Both are very smooth.

I did that test on all my cars. C300 and 2000 Lexus RX300 are both the same, smoothest. Next is 2002 Infiniti QX4, but smooth enough. Last is 2001 Hyundai XG300.

I went out today to try different C300 on different dealers. Yes their RPM drops to 500 but doesn't shake. I wonder why mine does. I'm thinking maybe from the fuel that the dealer filled on my tank when I purchased it? I haven't used up the fuel yet. Please advise.

put an octane booster in there and fill thank up with premium

The day I took the delivery, salesman filled the tank with regular gas (87, he admitted). I was not happy about it but the car never shaked.