Since 2020 Mercari forces an authentication process ( can take up to 3 days ) before any designer item is listed. “Let’s get this authenticated” pops up automatically during the listing process.They contract “Real Authentication” to verify authenticity. When you browse Mercari you’ll see “verified authentic” badge next to those listings. The seller pays for the service to get that badge.
I bought a “verified authentic” Chanel item and it was authentic. Would never consider buying a bag there without that badge though.