Informal business usage meaning "to act as an intermediary".
Definitionen
To act as an intermediary between two parties, especially for a fee or commission. The startup middlemanned the transaction, earning a small percentage on each sale.
Attributive noun usage; generally not gradable.
Definitionen
Relating to, characteristic of, or imposed by an intermediary between producers and consumers. The retailer promised to cut out middleman costs and pass the savings on to customers.