Don’t really trust eBay and gumtree. What’s stopping the phone being reported stolen and claim on insurance ?
Where would you suggest?

Not 2nd hand but if your also looking for a new contract I just got an iPhone 6 from mobilephonesdirect for £150 (64gb), with £42 topcashback making it £108
£32.50pm on EE 4g extra tariff (inclusive calls in Europe etc), unlimited calls and texts, 4gb data
6plus where only a few £ more
Best I could find by a long way, Three my existing contract couldn’t come close

I doubt you’ll find too many (any 6+) for sale yet and a genuinely unlocked one (not backstreet) will be worth close to retail price IMO. Any bought under contract won’t be unlocked by the provider without payment of a big surcharge as the contract will have been running less than 6 months, so sellers will use backstreet and then claim it’s fully unlocked.

Apple surely? All those buying it and realising it is too big and replacing it with the 6?

I already have one with o2. I need another as a gift for someone in Austria so needs to be unlocked and not on a contract.
Apple don’t have any refurbished.

Try looking in the classifieds on AV Forums.
There’s usually plenty of phones for sale on there.

Any more trustworthy than others?

On AV Forums I tend to just go by the Itrader feedback scores that people have. I’ve never had any issues with the classifieds there.