You will want to check:
- Shutter Count: The basic DSLR cameras are rated for 50000 shutter clicks with the Shutter unit. The shutter unit will probably last well beyond the rates limits, in any case, they are replaceable.
- Dead Pixels / Hot Pixels on Sensor: In rare cases, the sensor will develop pixel defects and since you are on an inspection spree, you might as well test it.
- Scratch / Fungus on Lens: Scratch will probably be absent on cameras maintained with loving care and fungus is a higher possibility in coastal areas of India only.
- Lens Aperture & Focus function: Are the aperture blades on the lens working smoothly? Is the lens auto-focusing? Is the focus ring firm yet smooth in operation?
- Lens Zoom Ring function: Is the lens zoom ring firm yet smooth in operation?
- Scratch, Dust, Dent on Camera: What is the cosmetic condition of the camera? Are the knobs, buttons, sliders, switches working as expected? Are the various sockets for output and tripod mount in good condition and working as expected?
- Battery Life: What is the current battery life of the camera?
Anything else I have forgotten? Like are you getting the box? the cds? the pouch? the cables? the original bill? warranty certificate?