traffic in (something)
To deal in the buying and selling of some commodity, especially that which is illicit or illegal. The mobster trafficked in drugs for years before entering into various other business models. The government has been accused of trafficking in relics pillaged from neighboring countries.
traffic in something
to deal in something; to trade in something, usually something illegal. Max had been trafficking in guns for years before they caught him. The president of the country was trafficking in drugs for years.
To engage in the buying and selling of some illegal or improper product or commodity: The police arrested the criminals who trafficked in stolen diamonds.