Beyond that, legally, anything you buy needs to have the seller invoke a concept called an invitation to treat[0] which in modern times is usually established at the point of issuing a receipt or tax invoice.

Even if someone has paid for something, the seller does not have to recognise that a transaction has taken place and would not have to enter or execute the contract. This is especially important in times where there might be a pricing error or something.

[0] https://en.wikipedia.org/wiki/Invitation_to_treat