[myrroddin]

I rewrote this function, then saw your reply. I was about to test it, and if it still goes into an infinite loop, I don't know how to stop it, given the delay to the server and back.

function PriceAnswer:CHAT_MSG_WHISPER(event, ...)
    -- stop listening to the event while we process the incoming message
    self:UnregisterEvent("CHAT_MSG_WHISPER")
    
    local incomingMessage, senderName = ...
    local outgoingMessageOne, outgoingMessageTwo = self:GetOutgoingMessage(incomingMessage) -- need to split returned prices into two; each message must be <= 255 characters
    outgoingMessageOne = outgoingMessageOne or ""
    outgoingMessageTwo = outgoingMessageTwo or ""
 
    -- stop possible infinite loops
    if outgoingMessageOne == incomingMessage then
        self:RegisterEvent("CHAT_MSG_WHISPER")
        return
    end
    if outgoingMessageTwo == incomingMessage then
        self:RegisterEvent("CHAT_MSG_WHISPER")
        return
    end
 
    -- debug prints
    self:Print("Outgoing message one", outgoingMessageOne == "" and UNKNOWN or outgoingMessageOne)
    self:Print("Outgoing message two", outgoingMessageTwo == "" and UNKNOWN or outgoingMessageTwo)
 
    if outgoingMessageOne ~= "" then
        SendChatMessage(outgoingMessageOne, "WHISPER", nil, senderName)
    end
    if outgoingMessageTwo ~= "" then
        SendChatMessage(outgoingMessageTwo, "WHISPER", nil, senderName)
    end
 
    if outgoingMessageOne == "" and outgoingMessageTwo == "" then
        SendChatMessage(format(L["Syntax: '%s N item' without quotes, N is an optional quantity, default 1, item is an item link or itemID"], L[db.trigger]), "WHISPER", nil, senderName)
    end
 
    -- we are done processing the incoming message, listen to the  event again
    self:RegisterEvent("CHAT_MSG_WHISPER")
end

And Dridzt was correct about hidden control strings. I had to write my own function, as I couldn't gsub things in TSM I don't know about. Writing my own function was a simpler solution.

-- TradeSkillMaster has some weird control characters in TSM_API.FormatMoneyString, build our own version
function PriceAnswer:ConvertToHumanReadable(num_copper)
    local gold_string, silver_string, copper_string = "", "", ""
    local gold, silver, copper
    if num_copper > 0 then
        gold = floor(num_copper / 10000)
        if gold >= 1 then
            gold_string = format("%d" .. L["g"], gold)
        end
        silver = (num_copper / 100) % 100
        if silver >= 1 then
            silver_string = format("%d" .. L["s"], silver)
        end
        copper = num_copper % 100
        if copper >= 1 then
            copper_string = format("%d" .. L["c"], copper)
        end
        return gold_string .. silver_string .. copper_string
    end
    return nil
end