A 22-year-old man had advertised a pair of Adidas 'Yeezy' 750 triple black SD sneakers online for $1,100.
He arranged to meet a prospective buyer in Johnson Park at Dulwich Hill, at 7:30pm on Monday.
The buyer tried on the shoes before pulling a knife from his backpack and forcing the seller to leave the park empty-handed.
Police said the thief may also be responsible for another theft earlier in the night.
The seller of a jacket, advertised for $1,000 online, organised to meet a prospective buyer in the nearby Arlington Light Rail Station.
The thief tried on the jacket before running away.
On each occasion the man has been described as Aboriginal or Torres Strait Islander in appearance and about 170 centimetres tall with a medium build and aged about 16.
At the time of the robberies he was wearing a black-hooded jumper and track pants.
New South Wales Fair Trading acting commissioner Andrew Gavrielatos said that while online shopping was a convenient way for people to interact, both buyers and sellers should be very aware of their personal safety.
"You need to be very aware that you are exchanging goods in an environment that you feel safe, perhaps having someone with you," he said.
"Be very aware of your personal details that you give out as well."