Customs officers searched Cox's Papamoa Beach Rd home and seized 25 knives also deemed to be prohibited offensive weapons.
Cox claimed he ordered the weapons to make money by selling to friends and advertised some on Trade Me after he had made legal checks on the internet.
Trade Me head of trust and safety Jon Duffy said Cox had been warned "on several occasions" for selling banned or restricted items before he was investigated by Customs.
The site had waited for Cox's legal matters to be completed before shutting down his account.
"This trader was on our radar and we were working with Customs to ensure the appropriate measures," Mr Duffy said.
He warned other people who misused Trade Me their online footprints could be traced by authorities if they were deemed to be doing something illegal.
Cox's lawyer, Genevieve Denize, told Judge Russell Callander her client had no history of similar offending and no interest in buying the weapons for unlawful reasons.
Cox, a solo parent, had intended to use the money he made to pay for his daughter's study. He earned about $2000 before costs, including the $600 to $700 he paid for the items seized.
"It hasn't turned out to be the money-making venture he had envisaged," Ms Denize said.
Customs lawyer Angus Senior said one phone call to authorities would have allowed Cox to clarify whether the items were legal.
"The only real purpose for the knuckledusters is to inflict violence on others and that is why it is not surprising that it is illegal to put them out on the open market," he said.
Judge Callander fined Cox $1000 plus $132.89 court costs.
Mr Duffy said Trade Me kept a 24-hour watch on the two million items listed weekly on the popular website.
The site uses software that highlights items listed for sale that may breach the law or Trade Me's terms and conditions.
Some knives and firearms are legal to sell and Mr Duffy said there were often only about two listings pulled from the site each month.
"We regularly work with Customs, and that's a key message for us to discourage people to list things (that are illegal). They leave electronic footprints on the site ... (and) all that information on the website could be accessed for law enforcement."
Mr Duffy said users had tried to sell some "wacky weapons" in the past, including ammunition from a rocket launcher (Trade Me, unable to determine if it was live, pulled the listing immediately), a home-made catapult able to launch concrete mixers, and police Tasers.
