A man answered an ad on Gumtree to buy an iPhone on Thursday, 6 September at around 8.30pm.
He agreed to meet the seller in Montgomery Court, Narre Warren South and after a short time was approached by the mobile phone seller.
As the buyer was about to hand over the money another five men have approached from nearby parkland.
One of the men produced a handgun and demanded the victim hand over his phone and wallet containing $1200.
The investigation into the incident is ongoing.