purchase of the entire supply of a commodity
• (n.) The act of buying the whole quantity of any commodity.
Co·emp'tion noun [ Latin coëmptio , from coëmere to buy up. See Emption .] The act of buying the whole quantity of any commodity. [ R.] Bacon. 
In Roman law, coemption was a form of civil marriage by a fictious sale of the two parties to each other.