More than 40 complainants felt the ad was offensive towards women by sexually objectifying them.
Some also felt the poster suggested women, like cars, were commodities to be bought and sold.
UlsterTrader.com said humour was part of its brand values and believed the poster would be taken as a tongue-in-cheek reference to what both sexes would regard as attractive attributes.
The poster appeared in 20 high-profile and high-traffic locations in Northern Ireland.
The ASA noted some complainants found the ad to be sexist because they felt the poster was targeting a primarily male audience and therefore implied that women did not need to buy or sell cars.
The watchdog noted that although the poster was distasteful, it did not intentionally exclude women therefore on this basis it was unlikely to cause serious or widespread offence.
However, the regulator did agree with the complaints relating to women being equated to commodities and considered the ad did degrade and objectify women by attributing cleavage to a car’s headlamps.
The ASA found the ad to be in breach of the CAP Code in relation to taste and decency and ruled that it must not be shown again in its current form.