If you're dealing with a lot of marketplace fraud, you might take a look at Sift Science (http://siftscience.com http://siftscience.com). We're a YC company tha

If you're dealing with a lot of marketplace fraud, you might take a look at Sift Science (http://siftscience.com http://siftscience.com). We're a YC company that uses machine learning to detect fraud, and we have a number of marketplaces using us successfully (including AirBnB, Uber and Kickstarter).