The simplest way seems to give those merchants a locked container they own and insert leveled items in the container. But I still have quite a few interrogations.
If I set the container as organic & respawn, the leveled items would basically reroll every few months (same time as the plants/supply chests). Is that right? But wouldn't the leveled items already reroll after 3 days, like dungeon loot? Can my leveled items still reroll if I set them to limited value like 5 instead of -1 for example? If I use the 2 calc options with a % of chance none, it should give the container a few items of the list but not all of them. Is that correct?
I'm currently looking at Sandman's H.E.L.L.U.V.A. Merchant Containers and the Mod Maker Manual to understand how it all works, but a little bit of guidance from a fellow modder would be great.