The major earning that the sites like Quickr and OLX gain is in the form of advertising revenue. There are two basic ways that these websites make money -
1. Throrugh the online ads placed along with the classifieds submitted by the members. The unique ad banners and ads from third party bring enough revenue to the site because of the higher traffic these sites gain.
2. These sites also let you place your ads as premium which means you pay a price for the ads placed by you. In return, they will give enough promotion for your ad.