I purchased a refurbished iPhone from a physical store in ?Mansfield via Ebay a few years ago and all fine. It was still within the Apple warranty period so offered some piece of mind. My Mum still has it so still going around 4 years later!
Plenty of e-shops which sell on there such as Envirophone who seem to have a lot of positive feedback and should be a good bet. No personal experience though. Not always the cheapest but at least come with some form of warranty from the seller.
My last phone I received via a WTB on here so always worth a go although I doubt someone will have 5 spare!