no necessarily; there is one thing in common, and that is the reliability of service,
the seller can not promise you something that can not fulfill. even in grocery store if someone wants to buy the last pack of apples, what are you going to do? either watch the guy or come back tomorrow,

looking at e-shopping examples, such as amazon, if there is no item  i.e. book available, they won't promise you one, all they do is asking you if you want to register for the item and they will let you know as soon as it becomes available.

at the end of the day, its just the system  engineers' decision as how they want their system to function, unless you can think of another solution,

the prerequisite for this use-case was that the system can not offer more than one of the available item, so in case two actors are using this purchase use-case, how your system is going to respond for this alternate?