If your PC came with a warranty you need to contact the seller or manufacturer about this. eBay will not get involved in warranty claims. Once their own 30 day money back guarantee has expired any warranty is between you and the seller, or whoever provided it.
Regrettably, warranties on eBay prove worthless. Sellers may claim a warranty, but when things go wrong many don't want to know. Even manufacturers' warranties are liable to be invalid for items bought on eBay.
If you don't have a warranty or are unable to get any action under it the best hope is that your payment provider e.g credit or debit card issuer may still be able to help. Otherwise, if the seller is a registered UK business seller you may have some statutory consumer rights - although enforcing them against distant, uncooperative eBay sellers may be difficult or impracticable. There is plenty of information online, or contact Citizens Advice. If the seller is registered abroad there's unlikely to be anything you can do.
Buying on eBay is not risk free. The user agreement even warns buyers that "eBay does not guarantee the existence, quality, safety or legality of items advertised, or the truth or accuracy of descriptions". Would you buy from a shop that said this? eBay is a jungle.
