[Phanx]

Edit: I guess I should refresh the page before typing my reply when it's been sitting on the reply page for 2 hours because I got distracted by work, but oh well. Leaving this here anyway.

On its own, "calling GetItemInfo" won't actually do anything:

/run GetItemInfo(6948)

You need additional code to catch the return values and do something with those. Let's say, for the sake of example, that you want to print those values to the chat frame, in which case something like this would probably work:

hooksecurefunc("ContainerFrameItemButton_OnModifiedClick", function(self, button)
    if button ~= "MiddleButton" or not IsShiftKeyDown() then
        return
    end
    local name, link, rarity, level, minLevel, type, subType, stackCount, equipLoc, texture, sellPrice = GetItemInfo(GetContainerItemID(self:GetParent():GetId(), self:GetID())
    print(format("|T%s:0|t %s", texture, link))
    print("   Quality:", _G["ITEM_QUALITY"..rarity.."_DESC"])
    print("   Item Type:", type)
    print("   Item Subtype:", subType)
    if strlen(equipLoc) > 0 then
        print("   Equip Location:",  equipLoc)
    end
    if level > 1 then
        print("   Item Level:", level)
    end
    if minLevel > 0 then
        print("   Requires Level:", minLevel)
    end
    if sellPrice >= 10000 then
        return format("|cffffd700%d|r%s |cffc7c7cf%d|r%s %d%s", abs(sellPrice / 10000), GOLD_AMOUNT_SYMBOL, abs(mod(sellPrice / 100, 100)), SILVER_AMOUNT_SYMBOL, abs(mod(sellPrice, 100)), COPPER_AMOUNT_SYMBOL)
    elseif sellPrice >= 100 then
        return format("|cffc7c7cf%d|r%s %d%s", abs(mod(sellPrice / 100, 100)), SILVER_AMOUNT_SYMBOL, abs(mod(sellPrice, 100)), COPPER_AMOUNT_SYMBOL)
    else
        return format("%d%s", abs(mod(sellPrice, 100)), COPPER_AMOUNT_SYMBOL)
    end
end)

You probably also need to tell the buttons to respond to middle-clicks, instead of just left- and right-clicks:

for i = 1, 13 do
    for j = 1, 36 do
        _G["ContainerFrame"..i.."Item"..j]::RegisterForClicks("AnyUp")
    end
end

If you're using a third-party bag addon that doesn't reuse the Blizzard item buttons, you'll need to modify its buttons and hook its click-handling function instead.