SecondMarket started dealing in private company shares on April 23, and after a slow start — 15 or 20 transactions in the first four months, with around $20 million in securities changing hands — saw business take off in the last part of the year.

Since September over $100 million in shares have been traded, according to managing director Adam Oliveri, and $320 million worth of shares are now listed for sale. Some of these shares are from top private companies — not companies whose shares investors may want to get rid of — and they’re being traded by top venture firms that traditionally invest in later stage startups.

Oliveri thinks part of the reason for the extra business is the hope that exits next year — both IPOs and mergers and acquisitions — will finally pick up. Based on the behavior of buyers and sellers at SecondMarket, he predicts digital media IPOs — maybe Facebook or LinkedIn or Zynga — because of the sharply increased interest in the shares of social media and digital gaming companies in the last four or five months.

He also hopes that business will stay good because perceptions of secondary markets are starting to change. “If you were a venture fund two years ago and somebody heard you were selling a secondary, they would figure you were trying to dump it,” he said.

Now, that’s not always true. Even though startups are taking longer to exit, LPs still need returns on their investments in venture funds, and employees who hold shares — originally the first people to call SecondMarket — get tired of waiting too. Right now 70% of the volume at SecondMarket comes from individuals and 30% from institutional investors.

SecondMarket itself is venture-backed — by NEA — and eats its own cooking, Oliveri said. Some of its shares are for sale on SecondMarket too.