When buying 2nd hand cams, one major consideration to look out for is always the shutter count (like mileage on cars) apart from cosmetic condition.
Note: Even brand new cams may have already over hundred shutter counts due to factory/customer testing.
However major brands do not allow you to check that unless you go all the way down to their respective service centres.
Nevertheless, with the help of some very useful (and FREE!) softwares, you can easily get those numbers out from your cams.
a) Opanda IExif - Based on image file and also able to display full exif (Exchangeable Image File Format) as well.
b) My Shuttercount - Check your shutter counts online.
EOSInfo - Connect the camera to your computer. Works on Canon DIGIC III/IV DSLRs (Check the site for actual supported models).
No software created so far for Sony Alpha.
Require some finger workout on cam itself.
1. Turn on the power switch
2. Open CF card door
3. Press "menu" + "OK" buttons simultaneously
4. Press the menu nav arrow arrow buttons in the following sequence:
- WB (Up)
- ISO (Down)
- Metering Icon (Left)
- AF (Right)
5. Press the shutter release
6. Press menu nav button AF (Right) again
The following will appear:
L - lens code
F - flash code
R - number of shutter activations
S - number of times a flash was fired.
C - number of times the mirror has been locked up (in cleaning mode).
U - number of times the camera has been turned on (ultrasonic wave cleaner activated).
V - counts two operations:
 number of times the mirror has been locked up in Live View, and
 number of times the AEL button has been used to auto focus while in Live View
CS - camera serial number
MCS - Is the same as the "Internal Serial Number" recorded in the EXIF data of pictures recorded with the camera. The 16-digit number contains a variety of information. Here is what we understand so far:
Digits 1-4 represent the camera model:
4001 = E-1, 4007 = E-300, 4011 = E-500, 4016 = E-330, and 4046 = E-510.
Digit 5 is the year of manufacture. E.g. "3" represent year 2003.
Digits 6-7 represent the manufacturing month. E.g. "06" equals June.