How does one buy Chachmah?
Rashi: He learns Torah.
Malbim: Buying with Mechir is unlike buying with money. The latter gives something that can be spent. One who buys with Mechir or barter, he trades a desired matter for something that he desires. Kesil is one who loves desire. When he comes to buy Chachmah, it will stop him from [fulfilling] his desires. It seems to him that he pays a Mechir for Chachmah. If he will keep the item of Chachmah, he will need to give for it the item of desire, which is also dear in his eyes.
What is the question?
R. Yonah: Why should a Kesil squander his wealth to acquire Chachmah? He did not yet acquire Chachmah of Midos! He did not fix his heart to expel bad Midos; anyone whose fear of sin does not precede his Chachmah, his Chachmah will not last (Avos 3:12). Great damage comes from one who learns Chachmah, and he has bad Midos.
Does he not have a heart?
Rashi: It is not in his heart to fulfill [Torah]. He learns only in order to acquire a reputation.
R. Yonah: "Lev" is acquisition of desired Midos - "v'Shome'a Tochachas Koneh Lev" (15:32).
Malbim: Lev is power that rules to block desires and go in the ways of Chachmah.
