GDI!! u cant writer properly ka?

lets help the guy
check the screen, check the serial number, check the lock, volume, silent buttons, the rest will be WYSIWYG

check the screen sensitivity..some of the 2nd hand iphones not so sensitive

You going to buy a brand new iPhone or 2nd hand?

Check for the physical functionality and conditions first i.e buttons are working well, no cracks or sign of open back covers, screen sensitivity, speaker working (play some songs if available), camera and stuff like that. Then check the software part i.e setting, wifi signal (try to connect to a hotspot and surf just to make sure the wifi working), cell signal (put in your own sim and try calling someone), Serial Number match from back cover with the one in setting or box (if have one and like Hanxz said) and errrr what more aaaa? The rest is up to you. It would be even better if you have someone that know about iPhone to tag along with you.

After you satisfied with all that only u give that person the $moolah$ (better COD la). The first thing you should do after acquiring the phone, restore the phone (to clear old setting and start fresh) and jailbreak/unlock yourself. If you do not know how to do that, you know where to ask.

I bought my iPhone second hand also. I just rest the basic like touchscreen, speaker, wifi, edge connection, camera and the condition of all buttons. If you are not sure just ask the seller the way to do it. Of course, call in and out, vibrate should to test also.

That's what I do when I get my phone. Because iPhone has no warranty even 2nd hand or new one so better be very sure

Better start with a fresh phone rather than a phone stuffed with garbage that you don't know off. You can get those apps back from installer but the cracked ones ... umm .. well ...   

won't. Brick so easily lah..lol

Make sure all function running and hardware condition first..

You use or try an real iPhone before this?

Damn noks... you drag me in... 

TS... if you are buying 2nd hand item by walk in or cod, maybe you can check the physical condition and functionality based on the particular given to you here rightaway to compare with the offered price... as for this stage, thats the only guide you have and what you should know... once that beauty is in ur hand, then talk again bout how to brick ur iphone in a fastest and effective way *lol*.... btw, suitable and competible apps can be dicussed later which is always self explanatory. e.g. "This apps is only for firmware 1.1.4"...

P/S: Are you sure you're wearing a right tag?