Montreal police are warning about a spike in muggings and assaults in the city's north end.

The victims all have one thing in common: they were trying to buy or sell online through classifieds sites.

Police say they have seen several dozen cases over the past year, many of them taking place in parking lots.

One assault victim was selling a $20,000 Rolex watch. The buyer wanted to meet at a jeweller to have it appraised, but the seller was mugged before being entered the jewelry store.

“The thieves spotted who was going to enter the jewelry store,” said Andre Durocher of the Montreal police.

Kimberly Ottoni said she believes she was set up for first time buying concert tickets online.

“I was texting him: ‘I'm wearing this jacket, I'm carrying this bag, so look out when you see me.’ I guess he spotted me from afar,” she said.

The seller was a no-show, and Ottoni is convinced she was targeted.

“On the way home, I was pickpocketed. My wallet was stolen with all the money I was going to use to buy the tickets: $150 dollars per ticket, all the money in cash,” she said.

Teens looking to buy cell phones or computers are also being robbed or attacked, explained Durocher.

“At this moment we have no reason to believe it’s a network, so that means they are single individuals that want to make a quick $400, $500 or $600 dollars, so that's why it’s a concern,” he said.

With millions of visits to these sites every month and reports of sales gone horribly wrong, Kijiji has posted safety tips for its users:

“We recommend a neutral public space, such as a coffee shop,” said Durocher.

Some police forces have been taking it further, with so-called ‘safe zones’ popping up at headquarters all over the country where people can meet to buy and sell items.

So far, that’s not part of the plan in Montreal, but police are urging greater caution.

“Don’t ever go by yourself and meet in a public space. Don’t ever invite anyone onto your home, as they will see what they'll be picking next,” said Durocher.

Police have made a dozen arrests and are working with buy-and-sell sites to track others down.