What's the best and cheapest way to trade in my iPhone 4s for an iPhone 5?

I'm currently not on a contract but wanting to replace my iPhone 4

Your best bet is to sell it on Ebay, as this will get you the most amount of money for your 4s. Then just put that towards your iPhone 5.

I would advertise it on a local selling place, OR go to a trade-in place such as CEX or something of the equivalent, you don't get much for them used now anyway as the 4S is long gone even in good condition. Amazon is better than eBay as it has less fees and lower fees.

Why not get a contract for iPhone 5?