Have a Wallapop decorated van to drive around NYC; with the words "Mobile Fleamarket" on the van. The van should be similar to an ice cream/food truck in layout. Drive through city and pull up in front of groups of people, asking what they want to trade in today and pull something from the truck to exchange with them! On the side of the truck can be a brief, easy step-by-step instruction to download the app. Also, have free Wallapop gifts to hand out; shirts, hats, etc.