David Richards registers on different platforms under different user names. On Mercari, his user name is summertrunks8820 (=what he buys, ie summer trunks, and date of registration ie August 8, 2020 - every time is a different user name). Then, he returns his old item and keeps your brand new item. Might be using different address, as the one on Mercari is his office address (real estate investment firm Richards & Robins)