"Online sales by way of buy and sell websites such as Craigslist, Gumtree, eBay, the Loot, and others are an alternative means to luring one's victims," an article titled "Just Terror Tactics" suggests.

"The viewing and collecting of the item should be arranged to be at the location where one seeks to carry out his operation. Upon the target's arrival, one can then proceed to initiate his attack."

Gumtree is urging users of its online classifieds to report anything suspicious.

"The safety of our community is our main priority and we urge anyone who thinks they've come across anything suspicious or concerning to report it to us," a spokesman said in a statement to AAP on Wednesday.

"We encourage all users to visit our Help section for tips on how to have a safe and successful experience and to be vigilant when using any online marketplace."