You can warp in and out of the Undergarden using Waystones, and doing so will completely reset the Stoneborn's trades and recycle them, even if you have traded and "locked" those trades in. I assume it's something about the Stoneborn not saving the fact it should have the specific trades or if they've been fully bought out.

I found while playing ATM 10, so this might be a modpack specific thing, but it is worth checking out.