From Law wiki, the wiki for law research

This case (Pharmaceutical Society of Great Britain v Boots [1952] 2 QB 795) demonstrates the principle that in point-of-sale transactions the offer for sale and acceptance of that offer is deemed to take place at the checkout, and that displaying goods on shelves does not constitute an offer for sale.

The Pharmaceutical Society brought a case against Boots for selling 'poisons' without the supervision of a registered pharmacist, contrary to the Pharmacy and poisons act (1933). Customers would place their intended purchases in a basket, then take them to the checkout for purchases. The plaintiffs argued that displaying the goods constituted an Offer, and placing goods in the basket an Acceptance of offer. This argument was not accepted the Queen's Bench or the Court of Appeal, as the cashier was -- by common practice -- free to refuse the sale, so the selection of good for purchase could not be construed a contract.