Case 1: A specific book is offered for 500 rupees and immediately accepted; the opening conditions are clear.
Case 2: The buyer leaves before accepting and later tries to accept the former offer; that offer has already lapsed.
Case 3: “1,000” is agreed but two materially different currencies circulate with no clear prevailing unit; the price is ambiguous.
These illustrate separate issues of formation, session continuity and price specification.