I suspect the problem is basically one of cash handling. In my limited experience if you allow any group of people to handle cash then some, or all will steal it.
I have spent a few hours positioning on a bus and listened, gobsmacked, to the cunning which was deployed in pilfering. A random selection of these disclosures are as follows:-
Hot chocolate - packet opened and split ebtween two or three cups. One packet's worth of receipts in till, the other two in pocket (ever wondered why your hot chocolate tastes so weak?)
Tea:- plastic cups retained and washed in galley, refilled with crew tea bags and re-sold over and over again.
The classic - selling crew water to pax.
Replacing crisps, munchies, flap ajcks with your own. For example flapjacks sold on board for £1.50 are bought from supermarkets for 30p each.
And on duty free trolleys - the absolute classic whcih I believe caused BA a huge head ache a number of years ago - replacing expensive brand named goods with cheap copies i.e. Gucci sunglasses for the cheap ones you can get for a couple of quid at market.
Generally these seem small time, but they are clever in that the stock sold tallies with the receipts. Well done all, here is your commission.
Only way round it is no cash purchases. Only done on a card.