As you're acting for a client I assume that you're not logging in the client's connected device. Most of the account crossover queries we receive here areis because two registered users are sharing the same conencted device, but failing to clear the browser cache between sessions.
It sounds as though you may have stumbled on an eBay security system, by which they're able to detect a login attempt when providing an account's correct login, but from an IP address that is associated with a different user.
As none of us know what security systems eBy employs it's hard to suggest how to overcome this. Maybe you could use a VPN to conceal your IP address, but this is almost treating yourself as a criminal!
