The UK government is looking into booking websites and its about time. In our experience these sites are nothing but ripoffs and we are trying to stay away from them, however there dominance in the market is making it hard to ignore them.
At The Old Manse in Kilmartin we presently do not use Booking.com, Expedia, trip advisor etc we only deal with direct bookings except from our airb&B entry. Air B&B charge us a % so guest coming to us through them are paying 9% more than people who book directly via our website. On trip advisor we now come fairly low on the list of recommended places to stay in Kilmartin, not because of the reviews but because we are not on their booking system and they get no commission from us.
We made enquiries regarding charges from booking.com its 18%, and expedia was 20%, with trip advisor being 21% these fees get added to your bill. The following article explains the situation quite well. article
At the Old Manse our booking system is called Free to Book its a small private company that supplies us with the means of an online calendar that we can show on our website and allow customers to book directly, we pay £1 per booking for this service. We also have an addon app to the system that allows us to take credit and debit cards online and at the the guest house.