You're underestimating the abuse, moderation, support, and fraud burden that results from doing so much sheer volume. So that's a lot of support people right there, and those support people will need a fair number of engineers writing the tools they use to do their job.
YEP. Every site that allows user-generated content has to deal with people who want to make stuff near, or past, the boundaries of what is legal and/or moral. If you move money around then you have to start worrying about people scamming each other, using you as a tax dodge or a laundry, and you have a whole bunch of fun new regulations to comply with regarding that sort of stuff as well.
The typical Silicon Valley Disruptor approach is to just wave away all responsibility except the absolute minimum the law compels you to do.
