The author is trying to imagine the very first time people traded one thing for another. The author means "For as long as people have traded or exchanged anything, they have known..."
However, the author does a bad job of it. Trade existed long before agriculture: we have seen trades performed by all 4 species of great ape (chimpanzee, bonobo, gorilla and orangutan). In addition, no one would trade a whole ox for six handfuls of grain.
The author was trying to make his or her writing more colorful and interesting by giving a concrete image instead of just speaking in abstracts, but he or she didn't take enough time to make sure that the image was believable. It's just bad writing.