It used to be that way when iphones kept their value better...
Maybe it's still true on the lower spec models... At most, I can only get about $700 Max for a pristine jet black iphone 7+ 256gb model, hardly more than the much cheaper 32gb base model. After fleabay and paypal take their (un)fair cuts, I'm down to about $600. You also run the risk of the ebay buyer ripping you off as fleabay will always side with the buyer, even when they ship you back a brick in a box.
You can try craigslist, but I'll be lucky to get $500 on craigslist, and run the risk of getting killed. Once you factor in taxes on your original iphone purchase, I'm basically down $500 or so in loss of value.
Many of the braggers here dumping their loaded phones for $800 are selling to unsuspecting co-workers/family members/friends who aren't checking ebay/craigslist prices.