Paramedics examined Fromong in a hallway but left without taking him to a hospital.
"He did not want to go out of here on a gurney," said his attorney, Louis Schneider.
Fromong has had four heart attacks in the last year. The lawyer said Fromong was "done for the day" but could resume testifying Tuesday.
Court resumed briefly with testimony by a witness from Palace Station, the hotel casino where Simpson, with a group of men, allegedly stole items from Fromong and another dealer last year.
Simpson and co-defendant Clarence "C.J." Stewart have pleaded not guilty to charges including robbery, coercion, assault with a deadly weapon and kidnapping.
The day began with jurors, who had been told to forget Simpson's past, being reminded of a civil judgment against the former football star in the 1994 slayings of his ex-wife and her friend.
"That's a different case and different facts, but the effect of the judgment is something you may consider," Chief Deputy District Attorney Chris Owens told the panel.
Owens spoke of Fred Goldman, father of slaying victim Ron Goldman, and said he would show that Simpson came to Nevada to confront the two memorabilia dealers he is accused of robbing because he felt that if he took back personal property in California, Goldman would seize and sell it.
Defense attorney Yale Galanter, angered by the tactic, told jurors: "This case ... is not about what occurred in California.
This case is not about Fred Goldman. It is about what happened in Las Vegas last year this time and whether crimes were committed."
After his acquittal in Los Angeles on charges of murdering ex-wife Nicole Brown Simpson and Ron Goldman, a civil jury found Simpson liable for $33.5 million in damages.
Galanter asserted that Simpson was only trying to recover personal items, such as pictures of his deceased parents, that had been stolen from him, and had no intent to commit a crime.
Fromong, who has said he was expecting to meet an anonymous buyer, testified that the group of men that preceded Simpson into the hotel room included a man who pointed a gun at him.
Fromong, who knew Simpson and sold his memorabilia, said Simpson appeared surprised when he recognized Fromong.
"He shouted, 'How could you sell my stuff? I thought you were a good guy. You stole my stuff,"' he testified.
Fromong conceded during cross-examination that some of the items he sought to sell were "heirlooms."
"He would give them to friends or family but he would never sell his own memorabilia," said Fromong.
The testimony supported the defense position that Simpson wanted to retrieve items of sentimental value and did not plan to rob anyone.
In his opening statement, the prosecutor described a recording of the hotel room confrontation which he said will reveal threats and force were used. But Owens acknowledged that most of the people involved had recorders, and one of them sold his recording to a gossip Web site.
Galanter said the price was more than $100,000. "They were in it for money. They are still in it for money," said Galanter.
The prosecutor only mentioned Stewart briefly in his opening. He claimed Stewart went along to provide a truck for transporting the materials. His lawyer, Robert Lucherini, noted that Stewart is the only co-defendant who has not taken a plea deal and that was because he has nothing to offer the prosecution.
"He didn't know the property was stolen, and ... he didn't know there were guns going to be used in that room," Lucherini said.
