999 stacks for vendorable miscellany

[LibitIncarne]

If I turn it into 24m gil, what am I meant to put in my retainer? Your suggestion of valuable things has been registerable miscellany, which wouldn't occupy any slots

Items exist to be used, not to be hoarded.
Put things you're going to need in your retainer, like housing items, crafting materials, relic materials, relic armor materials, Deep Dungeon/Bozja currency, potions, maps, weapons that you can't turn in for seals yet, totems that you can't get the mount or desynth the weapons from yet, bait that you'll need for later, materia you know you'll need for later gear, maybe eureka logograms/bozja memories that you'll take out when you go into the instance later.
Inventory management isn't that difficult unless you have hoarding tendencies.