Merchant inventories refresh after about 10 or 15 minutes real rime, depending on the NPC. If you check out the merchants in Ars Magicana, then head to Verdistis, then to the Dark Forest dwarven halls, the Ars Magicana merchants should have new inventories. That is what I did before heading to the wastelands, anyway, but check out the link below for an easier method.
If you quicksave before running your mouse over a merchant or hitting the alt key nearby, you can trade with them, and then quickload if they don't have very much gold. Merchants can hold on to some items you sell them, which stay at the top of the inventory list, so you may have to scroll down to check if they are changing, or not.
New Money Trick foundWelcome to the forum.