What to look for when checking out 2nd hand mobiles

The most important part is the hardware. Check if all the keys are working. Also check the call and if external speakers are present check if they are working fine.

You'll need to check all things like battery, charger, memory card, headset etc. If all are functioning properly.

Then check keypad, call quality, loudspeakers, camera etc.

get a box piece... else might turn out a stolen product

+1 try to get the bill also

All the points mentioned are correct. But keep in mind if you are looking for a specific model then checking out the second hand market can be a pain.
I have been hunting for a Nokia N90 from a long time but have not found even a single piece on the second hand market.

Purchased a Nokia 6120 Classic, 1 year old. Got the Box, Headset and Charger. This model is not released in India but I really wonder why. Check out its specs at the link below
*www.gsmarena.com/nokia_6120_classic-1958.php
Its price in the UK is equivalent to 8500 Rupees. The hardware matches the N73 except for the speakers and Camera and smaller screen at 2.0" but 16.7m colours. It uses MicroSD memory cards and is only about as big as the 3110Classic. It also has speed to match the S40 based phones.

I think always n must purchase a second hand mobile from a trusted person or what you ever seen in known hands.....never ever go for market or discounted ones.....I think buying a cheaper one will be good option instead of buying a crap one....