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Drugs, alcohol and cigarettes are among items being 'swapped' for goods advertised for sale on the internet.
And police are warning people to be careful about what they consider buying or trading on the net.
Michael Humphreys, 41, from Hei Hei, told The Star he advertised his Honda Accord for sale last month for $1500 on Facebook's Christchurch buy, sell and swap page.
The web page, which has 682 likes, has been operating since June 2011 and anything can be advertised for free.
Mr Humphreys was surprised when he was offered two bags of marijuana as a trade for his car.
"I was prepared to trade my car for a 32 inch television, not marijuana. I was shocked when I was offered it. After I wrote the post on Facebook, with a photo of my car, a 14-year-old replied and said he would swap his television for my car. I thought this was great but I didn't know he was 14 until an adult informed me. It made me question how safe these sites really are," he said.
Detective Inspector Dave Long said it is a serious crime to offer and/or supply marijuana.
"Depending on the amount being offered it would result in a fine or imprisonment. I am surprised that anyone would do such a thing on Facebook. It's the first time I have ever heard about anything like this happening before. It's a concern to police because of how unsafe these sites are. People need to be careful when they are using these pages especially to sell goods," he said.
The page is monitored by social network site members who report things to Facebook if they think people are trading illegally or breaking the law.
Facebook did not return calls to The Star about the issue.
Mr Humphreys said he first encountered the page because he wanted to advertise his car for free, instead of registering an account with Trade Me, which would cost $10 plus a listing fee of $39 and a $29.25 success fee if the car was sold.
"On Facebook you don't have any of these fees. But no-one is monitoring the page. People can post and sell whatever they like. It is dangerous especially given how easily accessible it is to children," he said.
E ciggys, cartons of cigarettes, cheap hair straighteners, mobile phones without chargers and mag wheels are some of the items advertised for sale.
People can also write on the page if they are looking for a specific car motor or item.
Mr Humphreys said the page and other similar pages were a potential target for burglars who stole to order.
"They are advertising products at ridiculous prices. They are so cheap and they don't care who they sell these items to. It's dangerous for children and for adults because of the risk it carries. You don't know if what you are buying is stolen or legitimate," he said.
NetSafe executive director Martin Cocker said while it's not illegal to list an item, depending what it is, for sale on Facebook, people need to take extreme caution using the social network site as a place to buy, sell or swap goods because there isn't a team monitoring the activity that goes on.
"People are not excused from trading rules if they decide to sell items on Facebook. If people are found to be breaking the law then pages are removed. It is helpful but it isn't a long term strategy. People are better paying the small fee of advertising on Trade Me because it is safer. There is a team that monitors activity and makes sure everything that's being sold complies with trading rules and regulations," he said.
Mr Cocker said there are a raft of penalties if people are caught selling illegal items online.
"Penalties vary depending on the offence. If you sell fake goods that company can take civil action against you or if sellers misrepresent items action is taken by the Commerce Commission. People also need to be aware of fraudulent trading - people saying they will ship goods but they don't. People are at a higher risk trading on Facebook," he said.
- By Samantha McPherson, The Star