Good to see that you're trying to get out ahead of it. It's unfortunate because as a small startup, you'd rather put your time into building the business.
But, fraudsters are relentless and they come in waves. You beat them back, all goes silent, then they come back with a vengeance. It never stops and they are inventive. Much of the fraud comes from outside the U.S. as well (China-big time in our experience), so there's virtually zero chance of prosecution. So, these guys have nothing to lose and everything to gain. If they get busted, they just don't get their money. It's all upside for them.
BTW, I used to think that raising the bar so they'll go elsewhere would be sufficient. It definitely helps, but it's not how it works. If there is any opportunity, there will be parties who persistently work you to find it, such that it will be a constant fight for you.
In any event, you are right that you will most likely be able to detect them at some point. It's just the cost/time involved and the damage to your brand/customers as they keep changing tactics. One thing you should assume is that whatever you put in place, they will defeat at some point. It is guaranteed.
Finally, services like siftscience also help, but they tend to just raise the signal and/or help you to corroborate suspicions. It can still take quite a bit of effort and invariably human interaction to interpret those signals.
Not that it can't be done. Just encouraging you to brace yourself for the fight ahead. While you may be starting out to build a marketplace, you will likely find that you spend more time in the fraud-detection business.
Good luck and kudos on your progress to date.