To sell stock to a casual customer select the red Create new button and select Walk-in Retail sale.
This will bring up the Walk-in sale pop up box.
Select the required product using the search field.
You can add discounts at this point (see miniguide Discount an Invoice) if required.
Select the Take payment button to go to the take payment pop up.
Enter in card or cash.
Select Print a receipt checkbox if required.
You will see this sale reflected in your reports but you cannot access the invoice after the sale is completed.