Webuycars selling cars that have already been sold

My family and I went to Webuycars at the Dome today to look at a VW Kombi. Upon arrival and after struggling to find a salesperson who could assist us to find this vehicle, were informed that this vehicle were at the workshop. Nonetheless, we found another vehicle (Ford Tourney) and requested a testdrive. The salesperson took us for a testdrive, and thereafter we decided to buy the vehicle. We started a finance application with the salesperson and raised our concerns that someone else might buy the vehicle while we were doing the application. He assured us not to worry and that the vehicle will not be sold to someone else. After finalizing our application another person (which I assume is his manager), informed us that he had bad news and that the vehicle has already been sold. How does this work ? How can you take a potential customer for a testdrive, and when he informs you that he wants to buy the vehicle, not mark it as "being reserved" or "sale in progress" ? After our testdrive, we immediately informed the salesperson of our intention to purchase the vehicle and proceeded with the application directly thereafter (lead nr 7FF***)