The risks of buying a refurbished iPhone
To be clear, if you're buying from a reputable refurbishing marketplace like Back Market, there is little to no risk in buying an iPhone, especially if you're backed by a money-back guarantee and warranty. The only thing you might consider is that a refurbished phone may have a slightly lower life span, but it's a negligible difference, considering properly restored iPhones can still easily last up to 6 to 9 years on average.
Unfortunately, the word refurbished does not have a universal meaning. Sellers on some websites may simply list any used iPhone as refurbished in order to raise their asking price, even when the device hasn't been repaired and tested. Others may repair some issues and check some of the phone's functionality, but neglect other elements. Many of these sellers have low quality standards and will leave you with an aged battery and an expensive iPhone that isn't up to par with an expertly renewed device.
If you're ever in doubt, make sure to check out the site's reviews to make sure you're getting a quality iPhone deal.
Avoid buying 'used iPhones' when possible
Keep in mind, if you're just browsing for used iPhones, you're likely to see a wide variety of conditions, which are not held to the same standard as refurbished. It might be tempting if you find a really low price, but used iPhones basically comes as-is and there is no guarantee that any repair work has been done to the device. Make sure you fully understand the difference between used and refurbished devices before buying from a seller.