Sometimes Skillet will not refresh the buy list when you visit a vendor. So it will re buy all the same mats it picked up the first time. Most of the time it is stuff you need, but you still have to manage it in your inventory.
If you mouse over the vendor buy button, it will pop up with the items it will buy. You should have a good idea if its close or way off. If you just purchased all but 10% of your items and you have to make a second trip. Pay close attention to that tool tip. If the full mat list is still there, it will buy the stuff you already got. It will do it in the same order as it did before.
So if you run out of gold, bag space, or what ever else and it does not finish. Take a second look at that tool tip for the next purchase.
Here is what I do to refresh it. Open up the skillet shopping list then mouse over it. If that window is open then the tool tip will refresh for me and the purchase will be correct.