How to unlock my old iphone 4s before selling it?

I saw on amazon.com that a used unlocked iphone 4s sells for a lot more than just a verizon one. I have never done this before so, how do I do it? Do I jailbreak, unlock, and reset the phone?

The original carrier can unlock if your off a 2 year contract.
if its a verizon iphone 4 it doesnt matrer if its unlocked, it only will work on verizon.

Verizon iPhone 4S phones can only be unlocked for international use. A US SIM will actually not work inside one.

You can unlock it with a $25 gevey sim only, not sure if it has bigger value then, as the new buyer can get the same kind of unlock. Factory unlocks for verizon are crazy expensive, unfortunately. :(

Check this trusted jailbreak and unlock guide for iphone 4s:
http://www.monitoring-softwares.com/how-to-unlock-iphone.html