Check how many of that item I have while shopping?

There isn't any way to see how much you have when looking at the merchant's item.

About the only thing in this area is the ability to 'pin' a crafting recipe or diagram which will then show you if the merchant has any of the required items.

I added this in line 570 of guiTooltipComponent:

if (m_playerInv.GetItemQuantity(item) > 0)
{
    addGFxItemStat( propsList, " ", "(owned: " + m_playerInv.GetItemQuantity(item) + ")                                                                                                                                                         ", " ");
}

It's a bit hacky because I couldn't figure out how to add text at that location without an icon another way.

As of at least the 1.31 patch, pinning a recipe and selecting an item in the merchants inventory that is required for that recipe will show you how many you have in your inventory out of how many you require.

For example, if I had 1 Mandrake Root in my inventory and needed 5 for a recipe, when I hover over the Mandrake Root in a merchants inventory after pinning it I will see a tooltip with (1/5) on it.

There is no way to do this in the game. However, it is achievable with help of a mod called Friendly HUD which adds many 3d markers to the game. And it also adds a feature about inventory. It shows how much of an item you have while in shopping panel, it also shows the number of items you own while in crafting menu, including the number of the items you have in your stash. Definitely the tool you need to help you with your concern. And it is easy to install by following the instructions as well as with help of some YouTube videos. Hope this would help you.