Vendors buy batches of items and they will only buy so much. They have a rather long cooldown until they will buy that item again. More than 10, less than 30 minutes. If you level the vendor it might offer better items or/ and give you more options to sell stuff to it. You level a trader by donating gold coins. I think it’s about 250-300 coins in the beginning then 500 and more for the next levels. To donate you use the calculator, don’t type the amount in yourself but press the numbers in the calculator and then press enter in the calculator too. Some items the vendor offers are far too expensive others are worth their price. If you level up a trader it counts for your whole house group. There is a bow maker in the town that sells you a boar saddle and hook arrows if you level himone time up. The blacksmith will sell you iron tools and a powerful iron sword if you level him.