Why would you want to be selling the windows 7 OS, you will surely need it in your new build?
You do not mention make/type etc of RAM or make of Graphics card which will make a difference. You also do not mention the age of your components either.
The mobo is probably worth £20-£25,
GPU is around £15-£20
CPU maybe £40-£50
RAM, I have just bought 8GB Corsaire XMS3 for £23 including postage from another forum so you might get around the same from EBay for your 4GB depending on type.
The PSU around £10-£15 though surely you will need that for your new build also?
Do not forget that when you factor in Ebay and PayPal fees and postage then what you will actually receive will be less.