So my friend sold me his Razer Blackwidow for 25 bucks, witch is a good deal, tell me if its not. And the only problem with it is that the S key seems to not work. My friend was not bothered to fix it. But is there any trouble shooting tips you guys could give me to fix the key. And some background is that the wasd keys are worn, So what could be the problem, the switch, the membrane or the actual circuit itself. And also if it is broken... Will buying a replacement switch fix the issue.
$25 for a Razer Blackwidow keyboard is good, for one with a broken key, not so. Although possible to change a key out, it will require striping the keyboard down and soldering. However, this is not your main problem. The main problem is if your keyboard uses Razer's own propriety switches. These may be hard to get hold of. Is the keyboard still in warranty by any chance?
Here is a link I found on the subject that you may find useful: http://deskthority.net/keyboards-f2/question-can-change-switches-of-mechanical-keyboard-t5723.html
If they're cherry switches, you could probably take the keyboard apart and solder on a new one.
http://www.wasdkeyboards.com/index.php/products/keyboard-parts.html
Keyswitches of various types available above.
If it's an older keyboard (2012 or 2013) it will use cherry mx switches. Chances are that your S switch is bad. If you take the keycap off and mash the switch down with your finger does it activate? If not then you'll pretty much have to take the entire keyboard apart, and desolder the switch and the LED, and then solder in a replacement from WASD or somewhere else.
Is the keyboard backlit? If it isn't then you might not have to do any soldering. You could just take the internals from some switch you never use (Scroll Lock, etc) and replace the insides of the S key with that. Look around online for guides on how to open up a cherry switch.