Everyone's intention is to profit, even the con artist, but the buyer and the seller are providing and receiving goods, meaning they actually have a reason to be involved in this specific trade. One party produces oil, a second party consumes oil, and the third party is just there to take his cut because he thinks he's smarter than everyone else.
It shouldn't be surprising when one of these parties isn't trusted by the other two...
It's essentially scalping. Do you trust the people who buy concert tickets in advance and sell them to you at a 100% markup? They're serving the exact same role in society as these market players. No one likes them for a very good reason.
