From the Global Settings (Home -> Members Area -> Selling -> Seller Tools -> Global Settings ) menu:
Enable Force Payment
If enabled, sales will automatically cancelled unless marked as paid within the set period of time.
Select product, add to cart, and in the Purchase Details menu see the following message:
Important: When item is not paid (or status set to Unpaid), the sale will appear as inactive. Visitors won't be able to see seller or buyer details and won't have access to the Message Board .