Honestly for an iPhone in good condition selling it for $270 OS way too low and a quick look says amazon trade in gives more cash about $320. The reason those are too low in my opinion is that an off contract iPhone new from Apple goes about $600-$700 so $450 you mentioned sounds reasonable. Once you go to the reset area of the settings app on iOS it wipes the phone clean of any personal info, then you see the iTunes restore on screen and you can feel free to sell it via eBay, amazon also offers a similar program(not trade in but actually selling it), or something such as craig's list.
But in the end the easiest way to sell it is to do a trade in program but you can get more money selling it yourself.