business partner; business associate; fellow trader
Refers to a person with whom one does business, often implying a relationship of mutual interest or shared trade.
He is a business partner of many years.
I started a new project with a business associate.
取引先 is a more formal term for a business client or supplier, while 商売仲間 implies a more collegial, peer-like relationship.
同業者 means someone in the same line of business, but not necessarily a direct partner or associate.
Compound of 商売 (business, trade) and 仲間 (companion, colleague).