Two things sadden me greatly: sellers who leave out basic information and those who say 'pickup only' without even checking out the mail options.
Firstly, weigh your system, bathroom scales are accurate enough. If you can't read the dial with the gear on then put a stand on the scales, adjust to zero then weigh your gear. Checkout Royal Mail Standard Parcels, their weight limit is 20Kg and few systems weigh that much. If you put 'this way up' or whatever on the box then there is a good chance that will be observed. Consider selling the speakers seperately, that's where most of the bulk and weight is.
Pickup only, you are selling to your county; mail it and you are selling to the whole country!
Many listeners are either still listening to, or moving back to vinyl records; if your system has a phono input then SAY SO! Aux. input? SAY SO!
TELL PEOPLE WHAT YOU ARE SELLING IN DETAIL.
TELL PEOPLE HOW YOU CAN GET THE SYSTEM TO THEM!