Q:

Trading goods with harpies

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)

A:

Collected treasures.
https://www.mnn.com/family/family-activities/blogs/little-girl-feeds-crows-in-return-they-bring-her-gifts

Then, in 2013, Gabi decided to do more than just share the scraps of
  her lunch. Each morning, she began filling a birdbath with fresh
  water, and setting out food — peanuts, dog food and general leftovers
  — for the birds to eat. It was then that the gifts from the crows
  started to appear.
Gifts brought to Gabi Mann, little girl who feeds crows Gabi's
  'treasures' include a blue paper clip, a Lego piece, a rusty screw and
  a pearl-colored heart. (Photo: The Bittersweet Life/Twitter)
Her collection also includes a miniature silver ball, a black button,
  a faded black piece of foam and a blue Lego piece. She stores the
  treasures that the crows bring to her in a bead container, with each
  gift carefully itemized and labeled.

The wild lands where the harpies live are inaccessible to humans.  But these lands were not always abandoned.  Other races and civilizations once lived there, and their relics and artifacts sometimes make their way to the surface, where they are collected by the harpies.  Crystals and dragon teeth lie on the surface waiting to be picked up.  Ancient magic and strange earth energies produce unusual things which could be spotted by sharp eyes in the skies.  
The harpies are collectors like the crows, but they are smarter and they know better what will fetch a price.  

A:

Information
Harpies can fly and this would make them invaluable to help map-makers, to plot roads, to inspect large fields, and any other number of activities.
They also could carry messages between the cities, creating a courier network that could surpass most kinds of treacherous terrain with ease.

A:

It is in this kingdom that diamonds are got; and I will tell you how.
  There are certain lofty mountains in those parts; and when the winter
  rains fall, which are very heavy, the waters come roaring down the
  mountains in great torrents. When the rains are over, and the waters
  from the mountains have ceased to flow, they search the beds of the
  torrents and find plenty of diamonds. In summer also there are plenty
  to be found in the mountains, but the heat of the sun is so great that
  it is scarcely possible to go thither, nor is there then a drop of
  water to be found. Moreover in those mountains great serpents are rife
  to a marvellous degree, besides other vermin, and this owing to the
  great heat. The serpents are also the most venomous in existence,
  insomuch that any one going to that region runs fearful peril; for
  many have been destroyed by these evil reptiles.
Now among these mountains there are certain great and deep valleys, to
  the bottom of which there is no access. Wherefore the men who go in
  search of the diamonds take with them pieces of flesh, as lean as they
  can get, and these they cast into the bottom of a valley. Now there
  are numbers of white eagles that haunt those mountains and feed upon
  the serpents. When the eagles see the meat thrown down they pounce
  upon it and carry it up to some rocky hill-top where they begin to
  rend it. But there are men on the watch, and as soon as they see that
  the eagles have settled they raise a loud shouting to drive them away.
  And when the eagles are thus frightened away the men recover the
  pieces of meat, and find them full of diamonds which have stuck to the
  meat down in the bottom.
The Travels of Marco Polo

So Marco Polo told exaggerated tales, but it shows like flying small creatures could go down mountains to the river beds to retrieve jewels or valuable metals like gold extracted by rivers and normal erosion.
Even if your harpies can't carry heavy loads, only a couples of ounces of gold would make a flight very profitable. They could even get sieves from the humans to get those jewels better.
It is even fun if they insist on bringing shiny but not very interesting rocks, like quartz, instead of dull uncut jewels like emeralds.

