
"Sir, do you have 10 items?"
"I have less."
"Great -- come to this line, then."
Did the man say "I have less items"? No. He said he has less. It could mean less than that, or less stuff or it could simply mean less. Be the annoying stickler if you like and criticize him, and the signs, if they say "less items" or "10 or less items," but "less" is a perfectly good concept and a perfectly good word on its own, and "10 items or less" is perfectly fine. Don't go inferring a supposedly implied word to create an error so you can criticize it.
I have plenty to complain about at the supermarket as people park their shopping carts in the middle of aisles and pay with checks in the 21st goddamn century; I will not be joining the chorus of fussbudgets insisting that signs be made silly and pedantic.