The purpose is a "shop gate", and the player will be charged for items he brings through. My preferred solution would be to mark all items the player brings in in the inbound gate as "owned by the player". At the outbound gate all items not so marked will be charged for. (I already know I can't mark all items IN the shop, or picked up in the shop, as the shop will include player crafting and the function to catch crafted items is apparently broken)

I can look for keywords set in CK on base items, but apparently can't set them to item instances? Is there any other way to tag (instances of) items?

Alternatively, the player could be stripped when entering, and the entire inventory counted for when leaving. But then I've been foiled by quest items, which the player can't drop and are counted for gold value like everything else. And I can't script-strip the player of everything including quest items, because the player will be bringing crafting ingredients into the shop (the ingredients brought in don't have to leave).

Is there a way to check if a item (carried by the player) is a quest item or not?

So, any advice anyone, please?