A Sydney pop culture fan facing possible jail time for making replica guns with a 3D printer claims he had no idea his hobby could land him in such strife.

Sicen Sun, 28, was arrested in February 2017 after he advertised one of his imitation weapons for sale for "$1 million negotiable" on a Facebook buy, swap and sell group.

Sicen Sun outside court on Monday. Credit:AAP

The replica SIG Sauer P250 pistol was advertised for three days on the group for military items, and two weeks later police arrived at Mr Sun's home in Waverley with a search warrant.

Officers found multiple replica guns, a 3D printer, and blueprints downloaded from the internet which detailed how to make the replicas.