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.