Hemant Lakhani, 69, a British citizen born in India, was found guilty on Wednesday of five criminal charges by a US district court jury in Newark, New Jersey.
Lakhani was arrested in August 2003 after a two-year international "sting" operation and accused of trying to provide material support to terrorists, unlawful arms sales, smuggling and two counts of money laundering.
His was among the first arrests of its kind on US soil after the 11 September 2001 attacks.
US authorities described him as a major arms dealer and the case as a significant blow in the war against terrorism.
Defence lawyers portrayed him as a hapless businessman caught in a set-up.
"Today is a triumph for the Justice Department in the war against terrorism," US attorney Christopher Christie said. "Mr Lakhani was engaged in a scheme willingly, knowingly and anxiously to sell arms to be used against American citizens."
Memories of 9/11 influenced the
guilty verdict, a juror said
"The evidence showed he was enthusiastic about what death and destruction might result."
During the trial that began in January, prosecutors provided videotapes and taped telephone conversations of Lakhani making the deal with an undercover FBI informant posing as a Muslim fighter.
They said Lakhani spoke admiringly of Usama bin Ladin and claimed his missiles could shoot down up to 15 airplanes.
Prosecutors also said Lakhani went frequently to Ukraine to buy Russian-made missiles he hoped to sell to an armed Somali group.
Juror June Cantor, 46, said after the verdict that the evidence was overwhelming.
"Today is a triumph for the Justice Department in the war against terrorism"
"It was black and white, there were films, there were transcripts, and in over 500 exhibits we could not find that he was reluctant or questioning," she said. "He was not entrapped."
Defence attorney Henry Klingeman said Lakhani was disappointed by the verdicts and said it seemed the jury was convinced by the videotapes.
"The best evidence and the only evidence was Mr Lakhani's own words, and that is what did him in," he said. Lakhani did not testify in his own defence.
The jury deliberated seven and a half hours in total.
Cantor said she believed Lakhani was motivated by greed rather than hatred of America and noted that the jury which decided his fate was influenced by their memories of the 9/11 attacks that flattened New York's landmark World Trade Centre twin towers.
Shoulder-fired missiles are seen
as a potential threat to aircraft
"It is hard not to separate your emotions because we are Americans and we went through 9/11 and he was so enthusiastic," she said.
Sentencing was set for 8 August. The most serious charge against Lakhani carries the possibility of 15 years in prison.
Lakhani was arrested in a New Jersey hotel room while he was demonstrating a sample shoulder-fired missile to an informant posing as a "Somali terrorist".
The prosecution said Lakhani accepted a $30,000 down payment for the missile and was eager to sell as many as 50 additional missiles.