The human country lies on a mountain plateau. On one side, the plateau stops at the top of a cliff, after which there are a collection of stacks and a group of tabletop mountains, none of them accessible with human technology.
Harpies live on the cliffs, the stacks, the mountains, and beyond. They don’t use either clothes nor tools nor fire, but are quite dexterous with their bird feet, and could probably do gardening work if they put their mind to it. They cannot lift an human; four of them could band together and carry a child, but humans won’t allow it, and the harpies (usually) oblige.
My question is : What goods/resources can be produced by harpies to be traded with humans?
It shouldn’t come from the harpy itself, so capturing one can’t get you anything. It has to be light, for harpies to carry it (so most metals are out of question). For one of the resources, I was thinking of a crocus-based dye. Some dyes were pretty valuable.
Sorry, I need to clarify a point. I'm not talking just about day-to-day local trade, but sought-after goods obtained through hard-won agreements (They don't look the part, but harpies can be shrewd, when they put their mind into it) with a noticeable impact on the economy. I like the answers so far, some of them will definitely make it in the story (provided I manage to finish it one day)