It's coming up to iPhone 7 time and you might be wondering what the best thing to do with your existing phone is. I would never advocate changing your phone once a year but sometimes the update is worth it, especially if you can get a good amount back for your old one.
In my experience the best way to get the most money but comes with potentially more hassle.
The easiest, just pop into a store with your iPhone and let them give you a price. If you are ok with a used phone then it might be worth holding on a few months until the new model starts appearing then you can trade your old one for the new model plus a bit of cash.
I've used several, most carriers have their own version too. I've used O2, EE and fonebank. The best option is to go to sellmymobile comparison site and take the best price.
I offer as guidance it's there, their prices don't match third parties but it's less hassle and you can trade at the same time as you get your new one.
The bottom line is if you are ok with potential come back then eBay or Gumtree are your best shots. If you can accept a lower price but no hassle then go for a third party online selling site such as musicmagpie.