The 32-year-old, named only as Christian L, used the pseudonym Dosensuppe ('canned soup') on the dark net and sold fully automatic, semi-automatic and military weapons over a 20-month period.
In his concluding statement, the judge pointed out that the man who shot nine people and then himself in Munich last Friday had purchased his weapon via the darknet; while there was no connection between Christian L. and the Munich shooter, he is known to have provided weapons to criminals.
Among his customers were a 21-year-old British man who planned a murder with a submachine gun, a Chechen who posed with a kalashnikov in front of an Islamist flag, and a far-right radical who had already carried out a bomb attack, SZ reported.
In the courtroom, he gave a statement saying that the Munich shooting had showed him how "dangerous and irresponsible" his actions had been, Suddeutsche Zeitung reported, and he added: "For the rest of my life I will distance myself from weapons."
Until recently, the dealer had claimed that he had only acted as an assistant to the arms dealer who used the 'Dosensuppe' account, and had occasionally sent parcels on his behalf.
However, investigators were able to show that Christian L., who is also an amateur marksman, had sold twelve guns and six military weapons, the Frankurter Allgemeine Zeitung reported. Prosecutors had initially claimed that Christian L. may have sold about 65 weapons on the dark net, an area of the internet where it is possible to remain almost totally anonymous, and can only be accessed by downloading a special programme.
The judge said that there was "no doubt" that the accused had been running "a commercial weapons trade" and that just those weapons would have been enough "for a small army".
Christian L, a trained optician who had been unable to find work, had boasted that he was "the largest guns and munitions dealer in Europe" and could provide all kinds of weapons within 14 days.
He was tracked down after customs investigators found a weapons package addressed to the defendant at Cologne airport in November 2014; it contained three gun barrels concealed inside a music speaker. He was arrested in October last year, and the conclusion of the nine-day trial came on Thursday, with the judge sentencing the weapons dealer to five years and six months in jail.
The judge said that the encryption of the darknet was so strong that the investigators had been unable to decrypt some of it, making the assistance of the defendant necessary.
Last November, a 20-year-old German was convicted of running a multi-million-euro drugs business from his bedroom via the dark net. Later the same month, a 34-year-old German arms dealer was arrested on suspicion of supplying some of the weapons used in the attacks on Paris.