Swap Iphone 3GS Plus Cash for Iphone 4S ?

I have an Iphone 3GS 32 GB unlocked to any network in white. the phone is in immaculate condition and comes with the box and all accessories. im looking to swap for an iphone 4s in either 32 or 64 gb plus cash so if you have one and want to keep using an iphone but need the cash get in touch