Why go through a middleman and use them as a messaging service when you can instantly call or email somebody directly?
Buying number plates via number plate dealers means you are paying the dealer's commission- this can add hundreds if not thousands to the asking price of a plate.
Buying number plates privately through Number Plate Classifieds means that not only do you avoid paying a hefty commission but you also stand to save money as you are negotiating directly with the seller.
Number plate dealers are good at what they do and they do serve a purpose and offer a great service... but...
They aren't for everyone and it is always nice to have the option to take control of things yourself and save some money.
By actually speaking to the person selling the number plate you are likely to get a better deal.
When you buy anything privately- such as a car - it's always possible to save some money as there is no third party involved and no fees or commissions.