I'm looking to resell my iPhone 6 before the 6S is released but I'm unsure what route to take.

In the past I've done eBay the the fees are quite high and had issues in the past with overseas buyers bidding (despite saying no overseas bids) so I'm reluctantly to go down that path again. I've considered using Gumtree but again woried about possibly frauders (always convinced I'm going to get scammed). So am I left with just mobile phone reseller sites? If so can anyone recommend who offers the best pricing for an iPhone?

Game give a fair price based on condition.

If its unlocked (only original owner can get phones unlocked) then you will get more.

Try CEX (webuy.com) or GAME or even Cash Convertors.

My local Game beats the valuations offered by competition such as CeX by 5%. Have a look at what CeX offer for your phone on their website, paying attention to what is offered for grade A and grade B. Pop into Game, showing them these prices and they will beat them by 5%. Remember it's up to them to determine the grade/condition. If it's mint and fully boxed with all accessories you should get a grade A valuation.

With a bit of effort you can nail down eBay to "no overseas bids" in the account settings.

Even with the very high fees (~15% including PayPal) I feel that I have always come out better from using eBay in the past.

Nothing wrong with selling on Gumtree either, just use common sense and a bit of caution when meeting people.