A Sydney man who made replica guns with a 3D printer and advertised them online is facing possible jail time but will have to wait several weeks before learning his fate.

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

He's since pleaded guilty to charges including possessing a digital blueprint for the manufacture of firearms, manufacturing a pistol without a licence permit, and possessing an unauthorised pistol.

The 28-year-old has an interest in cosplay - which sees fans dress in costume as their favourite fictional characters - and said he only ever meant for the replica firearms to be used as props.

They included a P90 sub-machine gun seen in Stargate and a MA5C rifle from the video game Halo.

Sun said he knew there were "grey areas" around the legalities of what he was doing - and it crossed his mind that the imitation guns could be illegal.

He claimed he never intended to sell the gun he advertised for $1 million and instead - like any craftsman - wanted his work to be acknowledged.

However, crown prosecutor Stephen Makin put it to him that he wasn't just doing it "for love".

Sun, who is the first person in NSW charged with possessing blueprints to manufacture firearms, said he has created a pamphlet and other material to educate others of the legalities of 3D printing props.

Judge Penelope Wass on Monday reserved her sentencing decision so she could further consider the objective seriousness of Sun's offences.

The case is next due in court on August 21.