So if the person in front of your stall has a stolen card, and you are about to sell them some goods. Let us say you are selling them a tv. They play with their stolen card, take the tv, the card owner sees a transaction they don't know and complain to their bank, you get a chargeback. You just lost a tv. In the UK this is rare, due to chip & pin. It is very easy to defend a chargeback when the customer entered a pin into your terminal.

If someone wants a tv delivered to their house you do a 'customer not present' transaction, which can use their house number and area code to authenticate that the card is registered at the address you are delivering to. Now if they raise a chargeback you can defend it by showing a delivery note etc. If you use a 'not present' to do a face to face sale surely you have just defeated the security model entirely?