Confused

What is the name of your state? Massachusetts
I purchased a used Mercedes from a Mercedes dealership. The car was certified looked great after 12 days of driving started hearing a noise. Question the previous owner and found out the car was involved in accident. Contacted the dealer for explanation and was told it slip their minds to inform me. I purchased a car fax report, but the accident was not reported. I found out the General Manager took the car in for trade and repaired it at their body shop. He then drove the car for a month before they sold it to me. Can a dealer do something like that?

Yes they can. Dealers are not required to provide a complete history of every car they sell. It is up the the buyer to investigate the purchase, this should have included an inspection by an independent mechanic. The inspection may have discovered that the vehicle had been in an accident. And just because a car was in an accident, it does not mean the vehicle is bad, I have a 93 Toyota pickup that was involved in an accident when it was a year old, I have now owned it since 97 and other than routine maintenance, it has been a perfect vehicle.