I get most of my iPhones from the refurbished section of the Apple website. Not great savings, about 20% off new. But they put in a new battery and put on a new outer shell. They're tested. One-year warranty. Free shipping and returns. You get all the accessories. The selection of phones changes regularly so check back if you don't see something you iike.

https://www.apple.com/shop/refurbished/iphone

Watch places like Slickdeals website. They will post $100-200 deals on older phones. I wouldn't buy used as most are selling for a bad battery or other reasons and even a battery is $100 to fix. You can get one new for that price but may have to go with the service for a year or buy unlocked.

GameStop. Really. Never would have guessed, but I asked at the Apple Store when I was buying a new phone for myself, and thinking of getting a used one for my son. Sure enough, went online and they had several of various generations. Ordered online, came in a few days. I liked that I could take it to a store f I had a problem, though I had none in the end.

BUYSPRY that's the spelling I just looked up my receipt!

I got my dd an IPhone 7 refurbished from the seller BLUSPRY (I think) on ebay-they have hundreds of thousands of good reviews and it had a warranty on it too. So far so good!