This is probably the most popular method of shoplifting today, its called the "the banana trick". What a person does is replace the barcode of an item with a barcode from a much cheaper item. Here is a broom which was shoplifted using this method. It can be tough to catch with your POS system unless your staff member is especially vigilant. One client of ours caught a shoplifter because the book title on the computer when she rung it up did not match the book being purchased. The shoplifter argued that he did not switch the label as it was labelled before he got it. He almost got away with that until her husband went over the security footage and discovered that he had come earlier that day into the shop, switched the label and then came back.
One solution is that we do market is scanners that measure weights and barcodes, these scanners would detect that the barcode of the item and the weight does not match. This does not always work as a bag of cheap bananas, and a bag of expensive avocados weigh the same.
We also do have rules based that allow you to put maximum amounts that you could reasonably expect on a stock item, this can often help.