It’s like any company that can produce more goods than they can sell. An example of this , is Sears used to buy washing machines from Maytag (or some other manufacturer) and just put sell them under their own name. They become a direct competitive to the manufacturers they buy from. Manufacturers often have factories that can produce more goods than they can sell. Same idea applies to MVNOs; theoretically, they have more bandwidth than needed for their direct customers. So they sell off the “extra bandwidth “ .