We joined Booking.com in January. Since that time, roughly 1/2 the bookings made are fraudulent-- that is, the "guest" books with an invalid credit card and a fake phone number. Usually these bookings are last minute--And, since Booking.com refuses to cancel the reservation for 24 hours, we lose out as by the time Booking.com allows cancellation, it is too late for anyone else to book.
This morning I woke up to yet another "booking" with an invalid cc and fake phone number. It is for 4/15, but I won't be able to cancel it until tomorrow morning, so we lose valuable time for another party to book.
These fraudulent bookings are so common that I no longer bother to block space for reservations that come thru Booking.com until at least 24 hours after the reservation. Eventually, this will probably result in an overbooking-- but I just don't know any other way to handle it.
I am frustrated that booking.com refuses to validate credit cards at time of booking or, at the very least, allow owners to immediately cancel bookings with invalid credit cards (if it is an error in entering info on the guest's part, they can simply rebook).