Gumtree.
Or eBay.
BUT for that i'd insist on Bank Transfer and make a deal with whoever buys it to mutually withdraw the auction bid.
This way you save some £ on fees and the bloke buying it gets it a few £ cheaper too.
For example (using made up numbers):
Sell it for £1000 on ebay, 10% fees = £900 to you all said and done.
Mutually withdraw the auction (only listing fees to pay then), knock £20 or so off ends you with £980 all said and done.
Benefits both parties then.
I'm going to have the same issue selling my Mac Pro, and ebay are not going to get the fees from it, that is for sure.