Cuban, 50, became “very upset and angry” in 2004 after the company told him in confidence it planned to sell stock below its trading price, the Securities and Exchange Commission said in a civil suit today at federal court in Dallas. Within four hours, he sold his 6.3 percent stake, avoiding more than $750,000 in losses after Mamma.com’s sale was announced, the SEC said.

“It is fundamentally unfair for someone to use access to nonpublic information to improperly gain an edge on the market,” Scott Friestad, the SEC enforcement official overseeing the case, said in a statement.

The SEC’s complaint focuses on a series of phone calls and messages between Cuban, the company and his broker. Cuban, who in 2006 started the Web site sharesleuth.com aimed at exposing “securities fraud and corporate chicanery,” vowed in a statement to fight the SEC’s allegations.