I traded in my white iPhone 5 at the AT&T store when I got my s5. They gave me $200 for it..... so basically I had to only pay for taxes to get my galaxy.=)
Go to Settings -> General -> Reset (at the bottom)
You can then choose what you want to reset, so just reset everything. It'll be as if it's a brand new phone and the new person will have to set it up with all your info being wiped.