This isn't about just ensuring that a conference is legitimate or not, it's a fundamental problem with presale events. You'll have the same issue if you have an ordinary merchant account as well, presales are a huge liability concern, especially for events, legitimate or no.

If you find yourself in circumstances where you require presale revenue in order to hold an event then I'd suggest using a crowdfunding platform like kicktarter or indiegogo or the specifically event oriented event tilt and picatic.