How much to sell an iPhone 3GS?

I have an iPhone 3GS that I got February 16th, 2011, it has no scratches, no problems whatsoever, its good as new, it has the box, the documents and everything, except the wrapper that was on the iPhone, and the wires.... but no earphones.... How much should I sell it for? Also, how would I go about selling it? I want to buy the new White iPhone 4... But don't know how, do I buy the iPhone 4 online, do all that stuff like tranferring my number through AT&T, and sell my iPhone 3GS just like that? How would that process work?
================
Sell Your Iphone

They are so inexpensive and last a number of years before they show any rust. Window film would not work for us as we also use it as a shower but that is an option WindowShades if it is being just used as a bathtub and your window sill can take the moisture.