The vehicle trading industry says more prosecutions should serve as a warning to illegal sellers.
Six people have been convicted of illegally trading cars in the past month.
It is illegal to import more than three vehicles or sell more than six within 12 months, without registering as a trader.
Motor Trade Association spokesman Tony Everett said while some offenders are unwitting, most are not.
"Unless, say, you inherited a deceased relative's car fleet or something, and you're in the process of trying to quit that, it's an entirely fair situation.
