” Speculators make their money by buying low and selling high;”
Selling to whom?
Each other, producers, consumers.
Do you really have to ask that? The obvious answer is, they sell to the highest bidder. Eventually a speculator must sell to an end-user who will take delivery; that happens when the end-user is the highest bidder for a particular delivery of oil. If there are too many sellers (speculators, producers, etc.) selling to too few end users, the price will tank and the speculators will lose money, unless they hold short positions instead of long positions. Do you really not understand this?