Well, that is only because I know for 320GBP in my country I can build a budget gaming build and there is no way you can sell me your computer for the price you want. The person you sell your computer to don't know about that. Work that to your advantage.
Sell your computer's strong point:
Pre-build and working HTPC class computer. Owned and maintained by experienced PC hobbiest. Silent and energy efficient, affordable and ideal for entertainment center for the family.
Equiped with 3GB of the latest DDR3 memory, this computer's memory is also expandable up to 8GB* for your future computing needs. *(with 64 bit OS)
Graphics card is bluray capable, with hardware acceleration to decode .264 video content. Freeing resources for the central procesing unit to perform other essential task.
The cpu with Intel hyperthreding technology ensure you a smooth computing experience even in multitask running demanding multithreaded applications.
Selling for 320GBP, this pre-loved computer is a bargin. I am selling this because I need this money for buying my new school uniform and graphics calculator for the new school semester.
Then you list your spec. Don't have to be too specific.
i5 3.2 GHz
ATI Radeon 5450
Something along that line, don't tell people you are selling it to get a better compter. That implies that what they are getting is inferior.
Think business like. Not sure who will buy it but... you just have to try. Man, being a sales person briefly was fun!