I agree with you regarding his high income in a low cost of living state (one percenting it in Arkansas- love it) detracts from a money motive.
At the same time, some, or many people, spend what they earn. A taste for bling could slowly, but steadily put him in debt, despite "one percenting" in Arkansas.
Then factor in that there do not seem to be any indications of another motive: Social media was not heavy on 2A references, militias etc. Was not known be a member of a militia, on a talk circuit (whether or physical or YouTube). Nor, did he engage his undercover customers with right wing talk or justifications.
Rather, his pitch seemed purely commercial- "I can get you more for less. "Dont see what you want on the table, just ask..." Even his selling point that no back ground checks were needed appeared to be largely a technical recitation of the law, sans any outspoken belief that such requirements are inherently unconstitutional.
This is not to say that M did not have any political beliefs regarding the Second Amendment. But... he seemed cash driven at heart.