Having run a marketplace, the scammers are just ridiculous. The one I remember most is that some group stole the credit card numbers of a church group to "buy" PS3s from fake sellers. Thankfully, they were really naive about it, so we reversed all the transactions before they hit, but that's just the tip of the iceburg.
So here's a question: so what do your customers use before they hit $10MM in ARR and get on your radar?