I think that September and October and also in the spring before York are two of the best times to buy used trains on the Buy/Sell Forum. Every year I see some great deals and this year is no exception. A lot of it probably has to do with guys raising York cash.
I remember not buying a nice set on the Forum in the hope that I would find a better deal or set at York. It didn't happen.
I have also noticed a lot of people who are downsizing or selling because of the economy. Seems to be a great time to buy.