There are no must buys. The vendors of skill books are where you will get 99.99% to 100% of what will want. However, nearly all of the NPCs have some items on them. You can trade with these NPCs like vendors, however, it generally isn't worth your time. Vendors reset stock every 60 minutes of time spent in game. By reset, they will restock any skill books you had purchased and reroll whatever other items they keep on stock. This allows you to revisit if two characters need the same skill book. Items sold to vendors remain in stock. This is great in case you sell something you didn't mean to. However, it can be difficult to tell the difference between the vendor's normal stock and what you have sold to them. As such, I highly recommend in each area try to find a vendor that has gold, but doesn't stock anything else you are looking for. Sell to this vendor to keep the inventory of the vendors you will be revisiting clean.
