Buying a used camera? Read on...

Disclaimer: This is not an exhaustive list. Do a much detailed analysis before you pick up one. These are some stuff that came across my mind when a friend asked. I jotted it down the points, and am putting it up here :)

1. Superficial inspection
1. Check if there are major scratches / peel offs. It means the camera is  abused.
2. If there are minor usage marks, it is OK.
3. Check if all the buttons are working properly.
4. Ask the previous owner to get the battery charged so that, you can take some test photos etc.
5. All doors( for the lack of better words! duh :-| )  i.e., battery, card, camera connectivity stuff like USB, video, remote shutter release etc., should be secure, no loose hanging stuff.
2. Clear any custom function if enabled before going to next step. This will help, else, can cause any unwanted doubts. Ex: when I was checking out a camera for my friend, the mirror lock up was enabled. I almost cancelled the deal thinking, shutter is not working properly :P. So clear all custom functions.
3. Shutter 
1. Check for shutter action, the shutter button should not be very loose. that said, it should not be very tight too.
2. Check at all shutter speeds from the slowest to the fastest works well. May not be at every individual step, but definitely check the normally used shutter speeds like: 1/80 to 1/250. Then check at its fastest, and its slowest. After this, you may check for one or two more randoms shutter speeds.
4. Lens 
1. Check if lens glass has any big scratches. There may be some dust scratches which is OK. big scratch is like when you have rubbed the lens against something hard in NOT OK.
2. Check if the lens action is smooth enough. 
3. It should not stick while moving the barrel (rotating the zoom ring).
4. Enable auto focus, and try focusing something close and then immediately far, the auto focus motor should be functioning and new focus point should lock. To achieve this, half press the shutter, it will focus.If the focusing happens right, then it is good :) (which is mostly going to be the case, unless the lens has been dropped some time.)
5. See, if there are any black/brown/green patches inside the lens, this is fungus, if that is the case, I would suggest you ask the owner to get it cleaned.
5. ISO
1. Shoot at all ISO values, not that this is a big test, but it is worth looking.
2. You may find noise in high ISO, it is OK, it is natural. Do not call off the deal for that! (well unless you are looking out for a Canon 1D Mark IV, or a Nikon D3x)
6. Aperture
1. This is a very important test for lens. Shoot from f3.5 (minimum f value) till f22 (maximum f value) in all apertures, it is quite natural no image will be seen at either extremes ( wide open , it will get blown out, narrow it will get darkened) that part is OK. We are testing the aperture blades here not the image.
2. To do the above, put lens on manual focus, else, it will keep hunting in low light.
3. When you shoot at all apertures, the aperture should not get stuck any time, if that is the case, the lens is ruined. Don't take it!
7. Flash (applicable for models with a built in flash)
1. Pull up the flash and see if it fires when u shoot.
2. Put the camera in auto mode and try to shoot in some low light, the flash should pop up by itself.
8. If the owner is giving you the memory card, format it there in the camera, and check the capacity. Ex: if it is a 1 gb card, it shud show 930 - 956 mb space. (rest of it is not available on any card its all the complex math + meta data)
9. Shoot in all modes of operation available. You should be able to see some images :)
10. If the camera has video capability, shoot video and check
11. Check for live view mode ( ask the seller to enable it if it is available and if you don't know how to do it ) it should function like a normal digital camera, with image in the screen.

• Remember, that the camera is finding another owner :) treat the camera well :D
• Do not low ball when you are negotiating, realize that the equipment you are getting deserves some respect. Not to mention the frustration you would cause to the seller by low balling the price.
• Do not compare the camera with a brand new one available on the shelf, (by this I mean, the looks and usage marks NOT functionality) remember this is a used camera. Functionally it should be in immaculate condition.
• Build a good relationship with the seller too. You never know, you may bump into him/her again sometime :)

Cheers,
Akshay