An indictment last month says Dani Tarraf hoped to ship the weapons to "the Resistance" in Iran or Syria.
Charges in the undercover probe include violating arms-control laws and conspiring to acquire anti-aircraft missiles.
Investigators say he was buying them for Hezbollah, a militant political faction in Lebanon.
Agents say Tarraf specifically wanted shoulder fired missles capable of bringing down an F-16 fighter jet, a staple of the Israeli Air Force.
Tarraf answered questions at a detention hearing Monday through an Arabic intepreter. U.S. Magistrate Henry Perkin found him a danger and a flight risk.
Prosecutors argued the thousands of weapons sought show Tarraf was not out for target practice but wanted to get them in the hands of killers.
If he's convicted, Tarraf faces a long mandatory jail term.
Defense lawyer Marc Neff says Tarraf has pleaded not guilty and understands the seriousness of the case. Neff declined the opportunity to request bail.
"He's looking at a mandatory 30 year minimum, up to life imprisonment," Neff said. "He's not going to get bail in the near future."
"He has no ties to the United States, and the conduct charge in the indictment involves very serious offenses, obtaining missiles for a foreign terrorist group," said Assistant U.S. Attorney Nancy Beam Winter. "I can't imagine a more significant danger than that."
Surveillance photos show Tarraf trying out rocket launchers and machine guns he allgedy ordered from undercover agents. Moments later, he was arrested at his Philadelphia hotel room.
It was a joint investigation involving federal agents and Philadelphia police.