Pick a system for how you shoot
Choose a camera together with its lenses and what you'll use it for. Weight matters for travel, autofocus for sport, and connections and recording modes for video. Megapixels alone won't tell you which camera suits you better.
Find out the lens mount and sensor format. Different systems from the same brand may not be directly compatible. An adapter can sometimes let you use a lens, but feature support has to be checked for that exact combination. Also check the battery and memory card type.
Weigh the test results
For a used camera, the shutter count is useful if it can be found, but on its own it doesn't describe the whole condition. Check what is said about the sensor, mount, controls, screen and ports. Signs of a drop or moisture can matter more than cosmetic wear.
Test shots and confirmed autofocus tell you more than a photo of the camera switched on. If you'll film with it, ask whether the video modes were tested too. A general «works» is not a test of every feature.
Count the whole kit
Check whether it's the body only or a kit with a lens. The charger, battery, body cap and strap should each be listed. Compare the total cost including the accessories you need to start shooting.