Problem with used phones: no guarantee of water resistance. Many of the resold phones got their battery replaced, which itself it a good thing, but they are usually not applying the gasket required for water resistence.
30+ years on this planet and I've yet to have any issue with water and electronics. Even if your phone died every single year from water damage it would still be cheaper to buy a beater every year than a new iphone every 3-5 years
Some of the best video I have from my honeymoon was taken on an iPhone while floating in the Pacific Ocean with my (new!) wife. I wasn't going to grab a GoPro or some kind of Ocean Case for a video camera, I had a phone in my hand.