Reliable Refurbished iPhone Sellers

Apple does not sell refurbished iPhones and never has, nor do they allow any Apple Authorized iPhone reseller to sell refurbished phones. The only refurbished devices are ones refurbished by third party companies. What those companies consider "refurbishment" varies wildly and you would need to ask them just what they do to their devices. Note that there is also no legitimate source of OEM factory new parts for non-authorized resellers or service centers, so any of these companies offering refurbished devices will be using used/salvaged parts, gray market parts or outright fraudulent parts.

Essentially any company or site selling refurbished iPhones is selling nothing more than used devices that they have cleaned up, or perhaps at last tested somewhat. Also, any warranty on such devices will be solely from the seller or the company that did the so-called refurbishment, but not from Apple.