-- cogs, springs and similar stuff for the dungeon tent, he sells on days that end with 0 (or multiples of 10, if you prefer): 10, 20, 30, etc.When does the merchant change his stock? The first time I visited him he had cogs, springs and other things useful for the Dungeon Tent, but after that he just has his usually weapons and doesn't show his other stock.
-- On days 1-9, and only on those early days, he sells skill books (of Light and of Sneak, in various format). Useful for later lives, but you probably won't have enough money for the first couple of lives.
-- Stating on Day 11 (except for the aforementioned multiples of 10), he sells weapons.
I seem to remember reading on Discord that the dev plans to have the merchant sell more stuff in some future builds, but I could be wrong