When I made that earlier example for you part of the reason I gave specific, late-in-the-line kinds a number called “bulk-identifier” was specifically to circumvent this problem. When you think about it, a single thing can have a lot of kinds. A vehicle is a vehicle, but it is also a container and a thing and an object. By manually defining a bulk-identifier for things I wanted grouped (and assuming the contents of a showcase would be strictly controlled to only include things with bulk-identifiers), however, I could create artificial exclusive “kinds” to derive any necessary information from.
I am trying to compare the number requested (number of entries in multiple object list is 2) with the number in the showcase (the number of things that match the noun), which is 1 for oil skein.
The other reason, and why I made buying multiple things only work for one kind of thing at once, is that you can’t be certain a player is typing “BUY 2 OIL SKEINS” and not “BUY OIL SKEIN AND ARROW”. The use of bulk-identifier and then comparing it to everything in the multiple object list was to prevent both of those from causing you to buy 2 oil skeins.