You'd need 211 more posts so not a fast solution but the Members Market on here is pretty good usually for shifting computer parts. I'm not sure that even if I knew of any places that buy secondhand computer parts I could post the location without breaking forum rules anyway.
I would possibly try in your local free-ads paper, word of mouth and/or noticeboards around work maybe? As for rough prices I'd guess maybe £30 for the CPU, £35-40 for the Ram and £50-60 for the graphics card. These are very rough prices though, you could take a look in the Price Check forum on here, I don't know if you need 250 posts for that as well.