I've got a platter from an old disk pack. It's not nearly as large, but it's cool because it's a "head crash" item. A 1/2 inch wide patch of the oxide is scraped away around the disk.
Back when I was a military contractor I was tasked with decomissioning a ton of disk packs from a DataPoint system we were eliminating. A group of us had to De-mil dozens of these following government procedure. The basics were to format each disk pack, run an application that wrote it full of all 1s, then all 0s then reformat it again. Once that was done we disassembled the packs and sorted the parts, the platters were then degaussed and sent to another unit which was responsible for destroying them physically.
We retained about a dozen of the platters after degaussing for awards and such.
Sorry for the long reply! Good luck on your auction!