To start the process, visit the new feature's portal. eBay will obviously need to know what brand and model your phone is, as well as what its specs are. It'll ask you if your device is in good condition, if it's broken and if it comes with accessories or its original box. Obviously, devices that are still intact and function properly will be worth more. You'll get the voucher as soon as soon as you decide to agree with the price eBay quotes, and you'll also get a shipping label you can print to be able to send in your phone.
Take note that you will need to upload pictures to prove that your device doesn't have a busted screen or any glaring issue if you say it doesn't. However, it's not entirely clear what would happen if eBay discovers a problem with it after you've already shipped it -- possibly because the program only accepts newer devices. While the service does sound convenient, it's only open to a select number of models, namely Samsung Galaxy S7 to S9 devices (and their plus versions) and Apple's new iPhones starting from the iPhone 6S 16GB to the iPhone X 256GB. They'll also need to be unlocked or under contract with AT&T or Verizon only. eBay will add T-Mobile and Sprint devices, as well as Google Pixel and select LG options in November.