Invaluable LLC, a Boston-based firm that operates the Invaluable.com online auction site and also sells online auction software and services, has raised $33.75 million in a Series D funding round.

Insight Venture Partners led the round, which also included existing investors Ascent Venture Partners and Commonwealth Capital Ventures. Bradley Twohig, principal at Insight, joins Invaluable’s board.

Invaluable focuses on the sale of fine art, antiques and similar collectibles on its auction site. The company, which traces its roots to 1989 but launched online bidding in 2009, attracts 2.5 million unique visitors per month and carries $4 billion worth of listings, a spokeswoman says. The site hosts more than 15,000 auctions annually and employs 80 workers. Last year, the company posted a 98% increase in gross merchandise sales compared with 2012, though Invaluable did not disclose those sales.

The new capital will go toward acquisitions, new technology and hiring.

“We experienced exponential growth in 2013, and this funding will help us continue to revolutionize the live auction industry even more rapidly,” says Invaluable CEO Rob Weisberg. “We are thrilled to be working with Insight Venture Partners, whose team brings additional expertise with respect to growing and scaling an online technology-driven marketplace.”

Invaluable says it recently signed a deal with eBay Inc. to make items from its catalogs available for online bidding by eBay’s users through eBay’s new live auction platform. In April, Invaluable announced a deal with EpaiLive, an Asia-based online auction platform for antiques and art. Catalogs from Invaluable auction houses throughout North America are now listed on EpaiLive’s platform.