First off, this is not a new item as you have it listed, it's a used item. "In good condition and only used for 6 months" is not new.
The parts you're selling only retail for new in the UK for about £1300. Knock it down for being a used PC and by someone unknown to the buyer and you're looking at realistically, a few hundred pounds less than half what you're asking.
Ebay is the best place to sell.