How do the websites like deals2buy, dealsofamerica work...i mean how do they get ROI ..or how do they earn money? because they don't sell anything themselves...also how can they post listings of other websites? like they have listings froom ebay, newegg, dell, lenovo, hp?

I am asking this because I wanted to start something similar to this...but don't whether i should or not? coz I dont know whats the profit