Hi My name is lisa, I am From UK, It happens usually flights prices changes in a sec, blaming someone for that might not be a good practice. I have booked with skyscanner and various other websites too It happens sometime.

Many third party agents do not have live pricing and in many cases the fare shown which you make a request to book is sold out or has not been available for some time. Even if you had not got a session expired error you would likely have found you would have received a call within 48 hours to advise that the fare shown was sold out and they require more money to complete the booking.

Often this is known to ensure that on comparison flights they appear to be the cheapest and get the interest of those looking for the best deal. Ultimately it is likely that the fares they ultimately have are the same price as booking direct with potentially their own fees on top, If you want to see live fares, the airlines website is the place to look.