NASH will handle all sponsorships and long-term commercial partnerships, as well as selling banner ads on the site.
The website, which had 1.3 million unique users and 16.8 million page impressions in January, allows users to sell goods and services online for free.
For a fee, members are also able to upgrade to a variety of premium ads for greater exposure.
Freeads is backed by Clive Jacobs, founder of car-hire outfit Holiday Autos, and is led by chief executive Duncan Horton.
Horton said NASH was an "obvious choice" of ad agency that would help his company achieve its growth targets.
"It has relationships with many advertisers and has mass-market reach, which fits in perfectly with our sales model," he said.
Horton said 4DS lost the business after it became apparent the two companies were going in "different directions".
"Where 4DS was going and where we were going was a slightly different angle," he added.
"NASH came in and they ticked the box on a number of levels for us."
NASH was founded in 2005 and was acquired by Media Corporation in November last year, providing further investment in its business.