The beginning - in the beginning of growing your marketplace, fraud should not even cross your mind. You have much more important challenges ahead of you, like how to get initial liquidity. The fraudsters will come, but only once you’ve achieved some initial liquidity.
The first fraud - once you have some transactions, you will soon also attract some fraud. The fraudsters are early adopters after all. They are always on the lookout for new opportunities and they are smart - they know that relatively new companies are porous, leaving ample opportunity to come in and take advantage of the system and the complete lack of controls.
The first fraud ramps up quickly - the fraudster community talks. Word spreads fast of vulnerabilities in new companies. Once someone has found a hole in your marketplace, they are likely to spread the word and you’ll see the number of incidents start skyrocketing. This is when you need to first take action. You’ll probably want to hire your first dedicated fraud controller. This is someone with experience tackling fraud at other large marketplaces such as Airbnb, Upwork, or eBay. All of those marketplaces have sophisticated fraud prevention departments that have observed and combatted countless attempts to take advantage of their systems. The number of professionals with true expertise in this area is quite limited, but you need someone with direct experience that has been in the trenches before.
The overcorrection - once you have a team in place, you’ll likely start to make product, process, and policy changes to combat fraud. You may enforce Facebook connect for identity, may even require driver’s licenses or background checks or other security measures. The key thing to understand here is that fraud prevention measures and growth tactics are often in conflict with each other.