A video game fan who 3D-printed replica guns for use in costumes has been given a 12-month suspended sentence in the NSW District Court.

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 for military items.

The post was taken down three days later, however the Firearms Squad launched an investigation.

Sun, then an advertising consultant, described himself to police as a "cosplay enthusiast" whose hobby of dressing up in costumes got "out of hand in regards to making the firearms appear as realistic as possible".