Check both condition and operation before buying:

• Test core functions: autofocus, aperture control, shutter speeds, single-shot and continuous shooting, and built-in flash if it has one.
• Check shutter actuations if possible. DSLR shutters have a finite expected life, so a higher count generally means more wear, even though failure timing varies.
• Shoot at the camera’s fastest shutter speed and review the images for uneven darkening, which can suggest shutter problems.
• Inspect image quality: take test photos, then view them at 100% on a computer. Look for missing areas, odd artifacts, dust spots, streaks, grease, or scratches.
• Check the sensor and viewfinder for scratches or contamination. A photo of a bright sky can help reveal dust or streaks.
• Inspect the lens mount for damage or looseness, and make sure it doesn’t allow excess light in.
• Verify ports and accessories: USB, HDMI/video out if present, battery, charger, and any included accessories.
• Assume an older battery may need replacement, but confirm the charger works.

In short: verify the camera operates correctly, inspect test images carefully, and pay close attention to shutter count and sensor condition.