[Kanegasi]

I use UnitName("npc") to get the name of the trading target for my addon.

local f = CreateFrame("frame")
function f.TRADE_SHOW()
    local name, realm = UnitName("npc")
    print(name..(realm and "-"..realm or "").." has opened a trade with me.")
end
f:RegisterEvent("TRADE_SHOW")
f:SetScript("OnEvent", function(self, event, ...) self[event](...) end)

I got this from http://wowprogramming.com/docs/api_types#unitID before it was deleted.

Edit: It's been quite a while since I found/used that, so I had a look in Blizzard's TradeFrame.lua, they use it to set the other player's name in the window:

function TradeFrame_Update()
    SetPortraitTexture(TradeFramePlayerPortrait, "player");
    SetPortraitTexture(TradeFrameRecipientPortrait, "NPC");
    TradeFramePlayerNameText:SetText(GetUnitName("player"));
    TradeFrameRecipientNameText:SetText(GetUnitName("NPC"));
    for i=1, MAX_TRADE_ITEMS, 1 do
        TradeFrame_UpdateTargetItem(i);
        TradeFrame_UpdatePlayerItem(i);
    end
    TradeHighlightRecipient:Hide();
    TradeHighlightPlayer:Hide();
    TradeHighlightPlayerEnchant:Hide();
    TradeHighlightRecipientEnchant:Hide();
end

UnitName is defined in the C portion of WoW's code, but GetUnitName is in UnitFrame.lua:

function GetUnitName(unit, showServerName)
    local name, server = UnitName(unit);
    local relationship = UnitRealmRelationship(unit);
    if ( server and server ~= "" ) then
        if ( showServerName ) then
            return name.."-"..server;
        else
            if (relationship == LE_REALM_RELATION_VIRTUAL) then
                return name;
            else
                return name..FOREIGN_SERVER_LABEL;
            end
        end
    else
        return name;
    end
end