Did they expand NX item trading?

These can't be combined to trade disappearing permanent-appearing Cash Shop clothing items, can they? Or more direct, should I be cautious in buying Nigiri?

You can dupe permanant Style box items, but you cannot dupe say....a permanant Trans hat. But you can trade it with hacks.

Because the Nigiri is a Style box item, It can be false-duped and disappear. So yes, I would be wary of buying one.