It's not that strange really.
There's a number of people who have access to a number of members cards, with the relevant credits on, who will happily sell on for as much as they can. You only have to see Facebook pages which pop up to get an idea.
The tour operators will have done some sort of deal with one or more of these. And the sellers can guarantee tickets to them, because of the predictable way LFC sell away tickets. The tour operator won't be bothered about paying £150-200 per ticket, as that's just passed on to the customer, the customer will be happy regardless, and the tout will be happy with this as they won't have to deal face to face with individuals, and will make a big profit in one go.
The only thing the club can do realistically, is deploy a couple of people in the ground as spotters, and get the ticket details from them.