Believe the 2012 e90 320d wld be in the range 60k to 70k. Mileage, i mean its already a 6 year old car. Assuming its a daily driver (which most d are) it may clock 20k mileage per year. do the maths..... 6 years times 20k km per year.....
But if the dealer is vouching for the mileage to be the original, you can check its service record book for service details. If not, note the registration number, and go to where the car was serviced (authorized SC) and tell them u want to buy the car and verifying the mileage.