By cpu I think you mean 'computer'. (Edit: its weird how outside of the industry, schools insist on teaching this, they do here anyway, living in theory land rather than the real world)
The CPU is a small chip which fits inside your hand.
Find out your CPU, RAM and video card details then compare it to others on ebay. I wouldn't expect a lot.
If you download and run CPU-Z, it will tell you what those 3 parts are (which are usually the most important when selling a PC).