Hi,
For buy now, it requires a user profile and that is why it is redirected to login. I understand that you might be looking for a seamless buying experience but it is required to map the buying transaction to an user. I think the best way to implement this would be to do following:
1. When visitor clicks buy now then it should open a small form to accept username and password and clicks buy now.
2. It would then create an user and redirects to paypal and let it finish the transaction.
Let me see if that is feasible. If it is not, then I would mention a caveat near the setting that “Allow users to bid” only works for bidding and not buy now.
Regards,