[Baertram]

You basically only need to know which items are the ones that count and mark them via an addon like ItemSaver or FCOItemSaver with a protection icon so you can see them at a glance (or filter them if you use FCOItemSaver) in your inventories.
This will even have them marked if you do a reloadui or logout etc.

From the addon code of GamepadBuddy I found that it checks for the quest name "The Covetous Countess" and then checks for these quest infos:

games
ritual
writings and maps
cosmetics
drinkware, utensils, and dishes

Depending on this text inside the quest steps it's setting a "wished itemtype" to some constant values which are used as the key for items that apply to this category:

-- Value Define
TCC_QUEST_GAMES_DOLLS_STATUES = 1
TCC_QUEST_RITUAL_ODDITIES = 2
TCC_QUEST_WRITINGS_MAPS = 3
TCC_QUEST_COSMETICS_LINENS_ACCESSORIES = 4
TCC_QUEST_DRINKWARE_UTENSILS_DISHES = 5
TCC_QUEST_UNKNOWN = -1

Items that apply for each category:

GamePadBuddy.CONST.TCCQuestTags = {
	[TCC_QUEST_GAMES_DOLLS_STATUES] = {["Games"] = true, ["Dolls"] = true, ["Statues"] = true},
	[TCC_QUEST_RITUAL_ODDITIES] = {["Ritual Objects"] = true, ["Oddities"] = true},
	[TCC_QUEST_WRITINGS_MAPS] = {["Writings"] = true, ["Maps"] = true, ["Scrivener Supplies"] = true},
	[TCC_QUEST_COSMETICS_LINENS_ACCESSORIES] = {["Cosmetics"] = true, ["Linens"] = true, ["Wardrobe Accessories"] = true},
	[TCC_QUEST_DRINKWARE_UTENSILS_DISHES] = {["Drinkware"] = true, ["Utensils"] = true, ["Dishes and Cookware"] = true}
	}

In function GamePadBuddy:GetItemFlagStatus(bagId, slotIndex) the addon checks each item for it's item tag if you got the quest active.

local numItemTags = GetItemLinkNumItemTags(itemLink)
    if numItemTags > 0 then
        local useful = false
        for i = 1, numItemTags do
            local itemTagDescription, itemTagCategory = GetItemLinkItemTagInfo(itemLink, i)
            local itemTagString = zo_strformat(SI_TOOLTIP_ITEM_TAG_FORMATER, itemTagDescription)
            if GamePadBuddy.CONST.TCCQuestTags[GamePadBuddy.CurrentTCCQuest][itemTagString] ~= nil then
                return GamePadBuddy.CONST.ItemFlags.ITEM_FLAG_TCC_QUEST, name
            end
            if GamePadBuddy:IsTCCQuestItemTag(itemTagString) then
                useful = true
            end
        end
        if useful == false then
            return GamePadBuddy.CONST.ItemFlags.ITEM_FLAG_TCC_USELESS, name
        else
            return GamePadBuddy.CONST.ItemFlags.ITEM_FLAG_TCC_USABLE, name
        end
    end

Basically it checks the item at bagid and slotIndex, builds the itemLink of the item, builds the item tags string (which is one of the constants mentioned above, e.g. "Drinkware") via function GamePadBuddy:IsTCCQuestItemTag(itemTagString)

If the itemtag string equals to one of the constants the item is a valid item for the quest.