[ZeroKnowledge]

You can use the "TRADE_SHOW"-Event. The name of the player who is trading with you is UnitName("NPC").

function WhoIsTrading_OnLoad()
  this->RegisterEvent("TRADE_SHOW");
end

function WhoIsTrading_OnEvent()
  if(event == "TRADE_SHOW") then
    DEFAULT_CHAT_FRAME:AddMessage("Trader: " .. UnitName("NPC"));
  end
end