Rashi: Anything that one exchanges in business, each party takes a different matter. One who tells his friend 'teach to me what you learned, and I will teach to you what I learned', [in the end] both of them have both [matters learned].

Malbim: This is like it says "Keno Chachmah Mah Tov me'Charutz u'Kenos Binah Nivchar mi'Kasef" (16:16). One does not find Chachmah from his Binah and investigation, rather, he receives it from Hashem. It is compared to Charutz, which one finds in depths of mountains. So Chachmah is like a Metzi'ah from the Owner of Chachmah - "Ashrei Adam Matza Chachmah" (13). Tevunah is acquired via 'business' - he understands and takes a matter amidst another matter - "v'Adam Yafik Tevunah" (13). It is like a monetary acquisition, which is sold and bought, like exchange. Monetary acquisitions are lost; they do not become part of the Nefesh. Chachmah and Tevunah are lasting, permanent acquisitions that become part of the Nefesh.

Rashi: It is a kind of gold.