It’s not long before the new iPhone is released so you may want to sell your existing one to help fund it.
I’ve bought and sold quite a few iPhones over the years so wanted to share my experience.
My advice is to use a company that specifically buys gear. I use Music Magpie who make it really straight forward. Get a price online then they even arrange the courier.
There are other companies out there but I’ve found these guys to be the easiest to deal with.
If you are ok with potential hassle for a bit more money then selling privately is another option. Avoid Facebook marketplace and Gumtree as these get the most time wasters in my experience. Selling to a friend or using eBay is probably the best avenue.
Personally I prefer to get a little less money and none of the hassle so use a service like Music Magpie.