Say an item in a checkout line is accidentally scanned and paid for by the person in front of the person who wanted to buy it. Say it's the last of an item, and the person who paid decides to keep it. Would the intended buyer have any claim over it? Like would placing it on the conveyor belt be a sort of non-verbal verbal contract to buy (though one-sided)?
Unless the venue has made atypical offers to customers, the understanding of a purchase at a retail store is that
- the placement of the items on the shelves for sale is an "invitation to treat";
- the presentation of the goods to the cashier is the customer's offer to buy; and
- the contract is not formed until the shopkeeper accepts that offer (See Pharmaceutical Society of Great Britain v Boots Cash Chemists (Southern) Ltd.  EWCA Civ 6 (05 February 1953)).
While there may be a secondary question about the mistaken buyer's entitlement to a refund, the intended buyer certainly has no contractual right to the item.