I dont think lvl up will make the merchant restock. Try buy something when you almost going to level up and check up on the same merchant to see if there any stock available once you gain a new level, especially weapons vendor.

From what I notice base on clearing the 4 flying fotress, after clearing each 1, merchant will show different goods compare to my last visit. And I dont necessarily level up in each fortress, since there not much exp gain actually, when 3x killing 2x enemy.