Quick tax revenue question

Hi all, we're about to file our 2016 taxes but I wanted to clarify something first about the revenue that we report. We are a marketplace, and for each transaction that takes place on our site, 20% goes to us. So, should that margin (rougly 20%) be the revenue that we report, or the topline revenue (the sum of the amounts of all the transactions that have taken place on our site)? Any clarity here is much appreciated.