How to trade my iPhone 4 for a 4s?

My friend gave me her iPhone 4 after she upgraded to the iPhone 5. I want a 4s but I can't possibly upgrade since I have no plan. It has minor scratches but the lock and home button do not work perfectly. It's 32GB though, and I wouldn't mind getting a 4s with 16GB. How coup I trade it? Or how much could I sell this iPhone 4?

Sell it

Take it to a local place that buys and sells phones? Many of them have a trade in price and a cash price, trade in your iphone 4, and buy and iphone 4s back from them, the difference in prics you'd have to pay shouldn't be too much.

Trade-in your iPhone 4 with NextWorth. They offer competitive prices for Apple devices, even ones that aren't working properly. Then, use the money you get to offset the cost of the iPhone 4S.