This addon can notify when:
Player A trades money to Player B
Player A trades money and item(s) to Player B
Player A trades item(s) to Player B
Player A trades money to Player B while Player B trades item(s) to Player A
Player A trades money and item(s) to player B while Player B trades item(s) to Player A
In trade window (bottom left) choose which version you want the trade notifications set to.
Original addon: https://www.curseforge.com/wow/addons/tradeshout
Overhaul addon: https://www.curseforge.com/wow/addons/tradelogunlimited