Checkout operators don't watch the screen
I did a stint at a large UK supermarket chain as a checkout operator as a summer job. A couple of things you all need to bear in mind.
Firstly, checkout operators are timed. You have to scan a certain number of goods per minute. If you regularly fail to achieve a pre-set average, you can get fired.
Secondly, when you are scanning items, you don't watch your screen. All you do is listen out for the "Beep"
Finally: How is a checkout operator supposed to know the difference between a £4 bottle of plonk and a £40 bottle of plonk ?
In our training, we were never told to read the screen to check what was coming up.
Providing the crooks don't put a goods to weigh sticker on a 42" plasma, and the goods are buried in a big trolley load of other stuff, most checkout staff wouldn't spot it.