So if the one seller counts on being able to buy the other seller's book if the first seller actually makes a sale, what happens if someone buys both listings a

So if the one seller counts on being able to buy the other seller's book if the first seller actually makes a sale, what happens if someone buys both listings at the same time and the higher priced listing is unable to source the item? Can the seller just cancel the sale for no fee? Why wouldn't everyone with a little time on their hands make 1.2x-priced listings for every single item available for sale on amazon and dropship from the cheaper listings?