This
article from 2009 provides some background:
At present, credit cards are swiped via wireless handhelds on aircraft but the transactions are processed when the aircraft gets on the ground. Because of this billing mechanism – which sometimes results in fraudulent transactions – there is a ceiling of the value of items that can be comfortably sold today.
The article goes on to mention that some companies were then beginning to offer real time in-flight card authorizations, but I do recall seeing a maximum purchase limitation on a recent transatlantic flight.
I don't think there is as much fraud risk as there might have been at one time. Think about the identification requirements to board a flight, especially the longhaul flights which sell duty free merchandise.