First of all, this is not a salesroom.
However I do realise you are trying to find out a value for you merchandise.
Breaking it down to separate parts, I`d visit E-bay and look at what the traders are asking.
i.e How much does RAM cost? How much does a particular HDD, or CPU or motherboard, or DVD or CD reader RW or whatever costs. Then offer them on E-bay, E-bid, or a local shop that will advertise for you.
I`d also like to know why you are selling such a PC.