Code:
* Id64ToString(*id64* _id_)
** _Returns:_ *string* _stringDesc_
* StringToId64(*string* _stringId_)
** _Returns:_ *id64* _id_
UniqueIds normally are unique, as the name says.
E.g. an enchantment, another level on an armor part will change the uniqueId, but not the itemId or itemInstanceId.
But the uniqueId is dependent on the item not the seller.
So you need additional info like the price and seller to see which item you check (if you want to make a difference there).