I recommend sellig your older PC through eBay because most of the community uses it and also you are able to compare the prices with searching for the same type of PC as you have and finding out the price and how much people would ask for it and you will also find out if it is really worth selling it. If you are able to sell it for £20 you could rather look for upgrading it or reusing some parts. Usually people buy older laptops/PC for parts so even if it is not fully working you can still sell it. Usually people buy those PC to get cheap parts and upgrade with a low budget. You could also look for any other websites where you are able to sell computer parts and laptops.