You can message an operator from their listing before you book, and from your booking page at any time after. Messages live in one place, so a question you asked before booking stays with the trip.
An operator's phone number and email appear on your booking page once the booking is confirmed. Before that, the conversation stays on BukNau so there is a record both sides can rely on — and so nobody can send you bank details that are not the ones on your booking page.