How to sell my iPhone 3GS on Amazon?

Break it down for me please? Like how to ship it and stuff

You don't.

^ Exactly. You don't!

Lol, here's why:
You sell it to an online site for a better price. For instance, your best known option is http://CashiPhone.com where you will receive an instant quote for how much your 3GS is worth and they will e-mail you the prepaid label to print out.So, shipping is free, and if you need help on the instructions they make it all clear and are quick to reply to your email. It's simple. And they pay you either through check or paypal, where you get money to be used on anything (not just amazon!)