DALLAS — The government’s insider-trading case against billionaire Dallas Mavericks owner Mark Cuban is heading toward the final buzzer.After six days of testimony, closing arguments are expected this afternoon in federal court in Dallas. The Securities and Exchange Commission sued Cuban, accusing him of avoiding $750,000 in losses by selling his 6 percent stake in a Canadian search engine company called Mamma.com Inc. after the CEO told him about a planned stock offering. The sale would have cut the value of his shares. CEO Guy Faure said that Cuban agreed to confidentiality on a phone call in 2004, then, when told of the stock deal, angrily said: “Now I’m screwed. I can’t sell.” Cuban disputes Faure’s account. The verdict could come down to whether jurors believe Cuban or the company’s CEO, a Canadian citizen who declined to come to Dallas. His testimony had been recorded on video and shown to the jury. Jurors also heard from bankers, stockbrokers and professors. New York securities lawyer Karen Bitar said the fact that the SEC’s key witness, Faure, testified by video is likely to help Cuban. The stakes are high for the SEC, which doesn’t take many insider-trading cases to trial and would be stung by a loss to a high-profile defendant. Cuban is not facing criminal charges, and even if he loses the lawsuit, he’s unlikely to pay much more than $2 million in penalties — about one-tenth of 1 percent of his reported $2.5 billion wealth.Cuban testified that he fought back rather than settle with the SEC because he did nothing wrong and refused to be bullied. His testimony may have helped his case. Known for his temper, he has been on his best behavior in court, never losing his cool under sometimes withering questioning by lead SEC lawyer Jan Folena.