Below is my guide on where you can sell your banknotes.
All around the UK there are collectors fairs which have a number of dealers who bring there collection for sale. They are always useful as the people who are attending are professional and organised. You are however unlickly to dicover a note cheaply which will turn out to be worth lots in the future.
There are thousends of banknotes for sale daily on ebay, although there are still a huge amount of online dealers who do not use ebay but instead sell direct though there webistes.