The plot is simply that aliens visit Earth and their interest in collecting cow pies (and just like other collectibles, special attributes make them more valuable) creates interest in humans in obtaining them to sell. Then the aliens leave and the market plummets.
I do not recall the name of the story and also at the end I think some new object is of interest which maybe someone remembers.
As silly as the premise sounds, if you look at what people really do collect and the amazing prices, the story kind of resonates. Digital crypto-kitties or whatever and more recent things selling for tens of millions or cryptocurrencies themselves seem not so different than cow pie collecting.
Moreover, the fundamental theorem of collectibles is that to make a huge amount, paradoxically the collectible had to not only start out as more or less worthless but also something people tended to use/read and then destroy or at least throw away. Old Nat Geo magazines are worth almost nothing because everyone kept them; Action Comics #1 on the other hand, well moms could not wait to toss those when their son went away to college or got married. The stories I have in just my own family.