you have to learn how to inspect a used car, dealers have tons of tricks to make a used car look perfect.

For body panel straightness need to inspect under flourescent lights.

For paint quality, swirl marks, touch ups, have to inspect in bright sunlight.

Interior plastics and leathers can be dyed and patched, so in bright sunlight inspect and use your sense of touch to feel for repairs on seat surfaces and and steering wheel etc.
.
Check and make sure all tires are the same.

You need to look at many C-classes to get a feel for what is correct.
Learn to inspect the fuse box in the engine compartment and side of dash to see what is proper, or what may have been a botched repair.

Lift trunk carpet and activate all devices to make sure everything works.
Listen for noise from vents in dash and learn how to use the multifuction steering wheel to check for stored malfunctions.

Key is to test drive and inspect numerous C-classes just like the one your interested in to learn what to expect.

You can also take the VIN number and attempt to ask the service department for a dealership repair history.