I buy and sell a lot of electrical gadgets on eBay and Gumtree. Particularly Apple.
You’d probably get one a bit cheaper on Gumtree as the seller isn’t paying any fees, and they seem to fetch higher prices in auctions. Apple stuff is very desirable on eBay.
But personally if I was buying I wouldn’t risk Gumtree as there is no buyer protection. I’d use eBay and pay a little more. Buying from a power seller who specialises in refurbished iPhones is a good bet. They also sometimes offer their own 1-3 month warranties with them too. Fortunately for you, if something does go wrong eBay are always on the buyer’s side. (Not so good for sellers like me who get scammed occasionally 🙁).
Check out how long the seller has been registered, the quantity of feedback and received and their percentage score. Also have a read of the feedback, make sure it looks genuine. Make sure isn’t all from very recently for lots of tiny purchases, and there aren’t lots of entries from the same eBay users for examples. Believe it or not people build up fake accounts this way and sell eBay accounts to scammers.
For example if you looked at me, I’ve been registered since 2003, have received 1025 feedbacks, and I’ve got 100% positive feedback. The feedback is for buying and selling, and spread pretty evenly over time. That’s the sort of thing to look for.
Actually I’ve just upgraded, want to buy an iPhone 7? 😉