You could sell it privately on eBay or Gumtree or you could cut out that hassle and trade it in. You might lose out on ultimate value, but it's quick, easy and an instant way to get cash for your existing iPhone.
If you're in the U.K., here are the places to do it.
Apple iPhone Trade-up program
If you're going to buy a phone directly from Apple then the easiest thing to do is to use the iPhone Trade-up program. Apple will give you up to £380 for an iPhone 7 Plus, and lesser values for other phones from there.
Apple rates aren't likely to be the highest, but if you consider the entry cost of the iPhone 8 Plus at £799 and the iPhone X at £999, top value for an iPhone 7 Plus would bring those prices down to £419 and £619 respectively. Decent for a phone you're not going to be using anymore anyway.
Away from Apple, there are a couple of national high-street retailers who will happily take your old iPhone from you. CEX is one of those and currently you could get more for your phone than from Apple, depending on its condition and of course, which model it is. CEX will also pay cash for your old phone, but you'll get less if you're not buying something else from them.
The thing with CEX is that they don't sell new phones direct from source, but each store will likely have a selection of 'A' grade or never used phones that folks scored on contract and immediately took there for cash. In my local store an iPhone X still in the original wrapper is fairly common, and so long as you register an account with them you'll get a 2-year no questions asked warranty on any phone you buy.
Similarly to CEX, GAME will also happily offer you a trade-in on your current iPhone. Rates will vary again on condition and model, but if you're an Elite member at GAME you will sometimes get an extra 10% value on your trade-in. The easiest way to check that is in the mobile app as the offers vary from month to month.
Equally, GAME will often have new or nearly new iPhones for sale that other folks have traded in. Including the iPhone X. Reward members will also get points for trading in or purchasing a phone from GAME, so you're earning towards your next PC or console gaming purchase at the same time.
Trading in is fairly easy to do but if you know you're going to want the newest iPhone every year, then you might also want to consider jumping on the iPhone Upgrade Program. This allows you to give back your current phone every year and get a new one for a reasonably monthly payment.
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