Hemant Lakhani, 69, a former clothing merchant from Hendon, north London, was found guilty of all five charges he faced, including attempting to provide material support to terrorists and the unlawful brokering of foreign defence articles. The offences carry a maximum sentence of 67 years in prison and fines of up to $2.5m (£1.3m).
Lakhani agreed to supply a Russian-made Igla missile to a government informant posing as a representative of the Somali-based Ogaden National Liberation Front. He was arrested in a hotel room in Newark, New Jersey, in August 2003 by undercover US government agents.
Lakhani had told the agents that the rockets could be used to shoot down 10 to 15 aeroplanes simultaneously on the second anniversary of the September 11 terrorist outrage, and offered a further 50 missiles.
Lakhani, who was born in India but had lived in Britain for 45 years, is heard on audio and video tapes praising Osama bin Laden and claiming the terror leader "straightened them all out" and "did a good thing".
"This case is about a man who enthusiastically tries to sell 200 shoulder-fired surface-to-air missiles to people who he believed would use them to shoot down planes in the sky with people aboard as part of a terrorist attack on the United States," Stuart Rabner, the prosecutor, told the court.
But Lakhani's defence team painted him as an opportunist amateur, with no known links to terrorist organisations, who fell victim to an elaborate sting operation in which the buyer and the seller of the missiles were in fact undercover law enforcement officials.
His lawyer, Henry Klingeman, acknowledged he was an unsophisticated "clown", but one, he said, who was utterly incapable of selling illegal arms without the help of a manufactured government plot.
"There was no missile plot until the government created one," he said.
"You can convict Lakhani, you can put him in prison for the rest of his life, but it's not going to make any of us safer."
But the prosecution insisted that Lakhani's ignorance was no excuse, saying he pursued the opportunity to sell the arms with "gusto".
"He's not charged with being a sophisticated criminal," said Mr Rabner. "You can be a dumb criminal."
Outside the courtroom the prosecution hailed the verdict as a victory in the war on terror.
"The evidence proved that Mr Lakhani was engaged in a scheme willingly, knowingly and anxiously to sell arms to people he thought would use them to kill innocent US citizens," said prosecutor Christopher Christie.
"He is a victim of his own evil, greedy, deceitful conduct."
Mr Klingeman acknowledged that Lakhani had "no compelling explanation" for his actions.
He would decide whether to appeal after sentencing, which was set for August 12.
"Mr Lakhani's very sad. He's an old man, his wife has been left behind."
His trial began in January, but took several breaks while he underwent an angioplasty for a heart condition and an emergency hernia operation.