[SDPhantom]

This is a more automated version of the script I use on slash command. It uses BAG_UPDATE_DELAYED to retry selling items as it fires once after all BAG_UPDATE events are done.

local EventFrame=CreateFrame("Frame");
EventFrame:RegisterEvent("MERCHANT_SHOW");
EventFrame:RegisterEvent("MERCHANT_CLOSED");
EventFrame:SetScript("OnEvent",function(self,event)
    if event=="MERCHANT_SHOW" then
        self:RegisterEvent("BAG_UPDATE_DELAYED");
    elseif event=="MERCHANT_CLOSED" then
        self:UnregisterEvent("BAG_UPDATE_DELAYED");
        return
    end
 
    local rescan=false;
    ClearCursor();
    for i=0,NUM_BAG_SLOTS do
        for j=1,GetContainerNumSlots(i) do
            local _,_,_,quality,_,_,_,_,novalue=GetContainerItemInfo(i,j);
            if quality==LE_ITEM_QUALITY_POOR and not novalue then
                rescan=true;
                PickupContainerItem(i,j);
                PickupMerchantItem();
            end
        end
    end
 
    if not rescan then self:UnregisterEvent("BAG_UPDATE_DELAYED"); end
end);