I think we could use another NPC like the Trader, but called a Collector (or something like that). Then we could sell unwanted items to the Collector (for tiny sums, because we all have a lot of something we don't want).

Then it would make sense that this Collector has a certain amount of supplies to sell when you visit him. So you cannot buy ALL the resources that you need. For example, the NPC might have 500 stone to sell, but when you buy it all, it would take some time for the Collector's inventory to reset.

This NPC could also have some neat cosmetic/decorative items to sell, but only 1 type available until the next time inventory resets -- there could be several things he sells and they'd be on a rotation, or even random every time the inventory resets.