When you do an order, we recommend you schedule your events first and then proceed to the Cart! This is because your event dates will carry over to the cart to check your Products Availability. Let us know and we can help you rearrange your Order Workflow Bar so your schedule comes before your cart.
When you open the Cart your default view lands on "ALL" which will show you what is available (in Green) or not available (in Red). If you switch to the "RENTAL" view, you can see how many are available against how many you own.
If you do try to over book you will receive a warning message from BPro which you can override if you wish. If you are interested to know what other orders your product is on during that event period, you can click on the little calendar icon beside the availability to see!