transactor; trader
A person or entity that engages in a transaction; used in business, legal, and financial contexts.
The transactor must sign the contract.
In financial markets, traders are required to make quick decisions.
トレーダー is a loanword often used for financial traders, while 取引者 is a broader term for any party in a transaction.
Compound of 取引 (とりひき, 'transaction') and 者 (しゃ, 'person').