Selling a used smartphone?
Finding a buyer.
Many online retailers and websites buy used phones. For instance, Gazelle
recently quoted me $149 for a used Samsung Galaxy S9 (64GB).
It doesn't cost anything to use online retailers with buy-back, or trade-in programs to find out what your device is worth, and if the price is right, you can usually complete the process and be paid within a matter of days with little to no trouble at all.
Selling your device yourself using sites like eBay may net you a little more than some of the buy-back or trade-in programs do, but it can be more of a hassle as well. You'll need to share a lot of specific details about your device, and hope the buyer is satisfied with the purchase in the end. Always be honest, and up-front about the specifics regarding your used device.