They are not sold out, they are waiting for the timetable for that day to be confirmed, before putting tickets back on sale.
Whilst the strike day timetables have been confirmed, those for the days either side of the strike days have not (yet).
The general message seems to be that trains will be rammed; it's a deliberate ploy to try and dissuade as many people from travelling as possible. I suspect that some short commuter services will be extremely busy, but I suspect that the London - Stafford ones won't be too bad. On the strike days, both the London - Liverpool and the London - Manchester services will stop at Stafford. The irony is that, whilst almost every place in the country gets no service or a very limited service, Stafford will actually have an improved Avanti service, at least between 08:00 and 15:00.