I would be surprised if you only lost £100 to £175 on sale of your phone. I bought a 6S 128 G yesterday for £450 pretty sure that was almost £700 when new. Interestingly it was a brand new phone manufactured in August 2016 with only 1 cycle on the battery. It seems some people selling their phones get them warranty replaced by Apple before listing. I could easily have afforded an iPhone 7 but after reading about problems with LTE connections, Problems with Bluetooth and BMW's, problems with various noises from the phone and without the "Courage" of Apple on the headset decided to save some money and get the 6S to replace my 6 which i had for two years.
Sure you can lose more than the 100-175, but I never have. Even if you look on say eBay just now (which is easily one of the worst places to sell it) and do a search for the sold iPhone 6s Plus 128GB, Unlocked, Space Grey and you'll find them for as high as, well I couldn't be bothered looking much, I'm far far too highly medicated right now to spend time looking through that, I've a ceiling to float across

but, £558 was one today. Obviously you'll get them down lower in the £400s, luck of the draw with an eBay auction.
Go to something like Gumtree and have a look through there. Same phone, same spec, used obviously not the new ones and you'll find them up to £575 (maybe more, again couldn't be bothered looking.) Again, there's the luck of the draw factor, but if you're not in a rush, hold out and you get decent money.
Good god you can even get up to £440 from somewhere like Geek Squad or Envirophone. Thats obviously a bit less, but thats selling direct to retailers, where you always get completely and totally ripped off. But the point is, if that's what they are paying for it you can easily add £100-£130 on top of what they offer if you sell privately.
I mostly sell privately, and I always get a really good return on what i paid for my iPhones/iPads.
Just for a bit of visual, here's a 2 second search on eBay, as you can see these sold today, used devices, not new, not warranty replacements, for up to £558. That's not at all bad, especially for somewhere like eBay. But if you must, then put it on there, slap a good reserve on it and Bob's your aunties husband.
***Edit
Sorry, there's one other thing I forgot to mention in all of this buying and selling malarkey. That's what too much morphine does to you, make you stoopid

... Eh incidentally, just for the record, not a junkie, severe nerve damage.
Anyway, one other thing I always do in this process is to use sites like Quidco. By doing that you can easily get £100 or more cash back, or sometimes a little less for your contract renewal year, that brings the initial cost of the phone down. Might as well run through the whole process....
During renewal year I'll also get some other mobile that I don't want with my new contract, but still has a decent value, and sell it on as well. So when you combine those you easily get £500-800 in cash.
That you can count whichever way you like. You can use it as part of your funding for your iPhones across the two years, thereby reducing the cost of each phone in the 2 year cycle. Or if you prefer you can use it to pay the contract for a year or so. I use it for iPhone funding as I'm paying for my mobile tariff anyway.
So essentially I work it as. Start of first year of renewal, sell old iPhone £550 ish, sell mobile from network, £500-£600 and £100 cashback That gives me up to £1250. That then pays for that years iPhone with about £350-£430 left over for next years phone.
Start of second year, sell my iPhone, £550 ish again and on average £400 left over from year one gives me £950. Buy new iPhone and have £130 left over for the next year.
Then the whole cycle repeats itself, over and over again. (Those sale prices are based on previous years and taking into account this years purchase price of course, the sale price should be higher next year with the price of the phones going up this year. And it may well be fuzzy in places, my apologies it's not easy to think properly in the state I'm in

)
But overall it's a fairly effective system for continued iPhone purchasing. And of course if your just starting out with your first iPhone, do the same. Get the cash back, keep it for year two and it helps with the second iPhone purchase. Then once you get to year three and your on your renewal, it all gets better.
That of course is if you want an iPhone every year. If you just want to to upgrade every two years you can just get a new phone from your carrier. You'll get less when you sell your phone because it's older, but you'll have less to pay to the carrier doing it that way anyway.
Right that's it I'm really done this time.
........ I think
