click here not auctions but worth a look and click here bear in mind many are base units so no monitor but if you look for a CRT monitor you can pick them up cheap.
Personally I'm not too keen on second hand PCs as you can get a brand new one with a monitor etc for £300.
Also worth considering click here
I think you're better asking about friends etc, some will be getting new PCs and will be looking for a home for their old one which might do you nicely and then you have some idea of how much use the PC has. A lot of the auction ones might be very hard used.
Good luck anyway.