There's no shortage of trade-in programs out there. All major carriers and several OEMs have them, and companies like Gazelle exist solely for trade-ins. eBay has just launched its own program, dubbed Instant Selling, and it thinks that it can beat out the other companies' values.
eBay previously had a program called eBay Valet, which would sell your stuff for you in exchange for a fee ranging from 20% to 50%. Instant Selling is different, though; it works much more like a traditional trade-in program, and it uses a company called CExchange. eBay says that its offers are up to 40% more than competitors'. Unfortunately, you're paid via an eBay voucher, not cash or PayPal. You'll also be charged a 10% final value fee, which all eBay sales require.
Another problem is the amount of phones currently eligible for the program. Right now, only Samsung phones from the Galaxy S7 to S9+, the 16GB iPhone 6s, and the 256GB iPhone X can be traded in. The phones have to be unlocked, Verizon, or AT&T models. eBay says that Pixel and some LG phones, along with T-Mobile and Sprint models, will come next month.