Some men in early Islam, like HAZRAT ALI (AS) were firm, sincere, willing to give his life for the cause of Allah. Persecution, torture, and threat to their lives could not separate them from the prophet, and that was what won the love of God and established God's religion.
In this verse, and in this bargain the seller is man (ALI (AS)) the buyer is God: The thing sold is human life; and the price obtained through the bargain is God's pleasure; what a nice deal!