A few things, but one is the PARTUUID shown in the lsblk and zpool status commands you were asked for earlier. Another is the metadata stored in the ZFS labels on each disk.
This is a tricky one. If these drives were new when you bought them, and if you have always put them in your system in the order you bought them never leaving one sitting on the shelf, one way would be to look at the Power_On_Hours of all drives and sort them from oldest to newest. Then cross reference with your invoices, oldest to newest.
You can also ask the seller(s) if they recorded serial numbers of items they sold you. Some do for purposes of validating returns and such.