First I would take a look at Craigslist and look at how much they go for in your local area. Then I would check out see how much they go for on reputable sites such as Swappa. Depending on the condition of your phone you could add or lower the price of others being sold.
I use Swappa a lot and just want to add that almost everyone will ask you to lower your initial price. Just keep in mind before lowering it that Swappa gets a little piece of the sale and that you would also have to pay for shipping.