Worst service ever on booking accommodation

The worst service I ever had. I booked through booking.com as the lady informed me over the phone. I luckily did not pay anything however did confirm with her if I should make a payment in advance. She informed me via email that is it not necessary I can pay on arrival. I phoned yesterday to confirm that everything is still in order for the booking as she confirmed yes everything is fine. About an hour later I received an email from her stating that she has no rooms available for us anymore. The excuse then was other people booked and paid. I did ask her why she first told me I can pay at arrival if that was the case. Someone else phoned to request if she has available for the same dates and she said yes she has 6 rooms available. How is this possible??? I will never book with this guesthouse again.