Buying a brand-new iPhone can be quite expensive. However, there are other options, and purchasing an iPhone does not necessarily have to break the bank as there are numerous options for refurbished devices. We take a look at what older iPhone models constitute a good deal and what to look for in refurbished devices.
https://www.notebookcheck.net/Buying-a-Refurbished-iPhone-An-Affordable-iPhone-Risk-Benefit-Analysis.537300.0.html
In the U.S., I've had good experiences with Swappa. They have both private sellers, and companies with a reputation to maintain. The buyer can decide if they'd like to take the risk of dealing with a private buyer (prices tend to be less), or go with the company option. Either way it's much safer than dealing with an auction site.