If we build a ticketing platform like Eventbrite that rewards users for sharing sales, do we need an MSB license?

As long as you are not holding on to the payments in your accounts, you would not be classified as an MSB. You should not be touching the money at any part of this process chain, except for the BSA regulated FI, to push your commission (intermediary charge) out to your bank account. Your processing has to be straight-through.

If you touch the money at any point in time, you'd be a money transmitter.