A 1999 Centre of Retail Research survey estimated that up to 16% of customer theft was linked to staff collusion. What's more, staff see collusion as much safer than other forms of fraud - they don't get caught with the goods.
Collusion can take many different forms. Staff might sell an item at a ridiculously cheap price, if auntie Joan needs a new leather jacket she can have it for the price of a pair of socks as long as her nephew on the till knows the product code for the cheaper goods.
Fake cash refunds are another popular scam, you could try to iron that problem out by making sure all refunds are double-checked and countersigned by a supervisor.
Then there's skimming, where the person on the till conveniently forgets to enter in a couple of expensive items.