My understanding is buymobiles.net is a large and reputable company (buymobilephones.net forwards to buymobiles.net), although I have not used the myself.
If it was me, I would spend the money instead on the brand new Samsung Galaxy S6, out 10 April. It is by all accounts a fantastic phone. Incredible display, fantastic camera, superfast processor, quick recharge, wireless charging, beautiful case design and colours, mobile payments ready, fingerprint touch security. I would say it's a much better phone than the now slightly dated iPhone 6.