A Mawson Lakes man who attempted to sell more than 20 used cars online and lied about some of their backgrounds has been fined $8100.

South Australia's consumer watchdog, Consumer and Business Services, took action against 30-year-old Aaron John Hindle, charging him with two counts of carrying on business as a second-hand vehicle dealer without a licence and two counts of making false and misleading representations.

The Elizabeth Magistrates Court heard that between March 2019 and June 2020 Hindle tried to sell 23 cars online - including a Toyota Aurion and a Mazda Tribute - on sites like Gumtree and Facebook.

When advertising the Aurion on Facebook, he claimed the car was a "one owner car" that had never been used as a taxi, when it fact it had three previous owners, and had been used for a taxi for at least two years.

Hindle had advertised the Tribute on Facebook saying he was selling it on behalf of his parents when he'd purchased it from someone unrelated the day before.

Acting Commissioner for Consumer Affairs Fraser Stroud said the offending was serious, especially since Hindle had been warned by authorities that he needed a licence if he intended to continue selling second-hand cars.

"There are protections available to consumers under the Second-hand Vehicle Dealers Act that simply aren't available when a used car is sold privately," he said.

"Before Hindle came to our attention, he'd been using fake names and licence numbers to avoid detection."