This has changed:
You can now buy tickets to Prague on the intercity.pl website
If I have to book a ticket when I arrive it's likely to be far more expensive and possibly the trains might be full ( no idea how popular the route is ).
There shouldn't be a problem.
Does anyone know where resellers get their tickets from? How is it possible that they can buy tickets for a journey with an end destination outside Poland, but members of the public can't?
They have a contract with the rail company - but see my first comment.
Am starting to wonder if it might not be easier to just fly from Warsaw.
Very cheap if you book in advance - I just did this. Use Momondo.