If you are having trouble connecting Vendoo to Mercari, follow the Mercari Connection Checklist:
Troubleshooting Mercari Connection Issues in Vendoo
Make sure that you are logged into Mercari on Chrome, and visit the marketplace connections screen to connect your account.
Are you a brand new Mercari user? Try to list your first item on Mercari directly. Then, repeat the instruction above.
If the problem persists, please contact customer support.