Firstly, I see no trouble in regards to this thread, the Karoo forums can be something asides a discussion area for various service providers in-which can be both good or bad. If this was not the case however one could only imagine that this community would become somewhat of a dull, repetitive and unappealing place to become a member of. In regards to your computer, I would consider yourself lucky to even have a buyer that is willing to spend money on something that is second hand. Second hand computer part's for instance can be more-often-than-not damaged through either general or excessive use even without the users' consent, when dealing with used I.T component's one must realize that a possibility of serious fault's may start to accrue. These faults will obviously be related more-towards a hardware level where-in money has to be spent. As by this point I suspect that you are beginning to understand, this would not be the case with something new opposed to second hand and could possibly result in a cheaper long term solution.
Here is my view on this machine estimated price:
CPU: AMD Phenom 2 940 Quad Core (Price new £95 from various retailers, second hand I would estimate at £45 depending on history i.e over-clocking est.)
GPU: Nvidia Ge-force 285 GTX OC (Price new £195 from various retailers, second hand I would estimate at £110. However, even if indeed factory over-clocked this does not mitigate the fact that your VGA card may be damage as a result).
RAM: 4GB DDR 3 1066/1333 ?? (This price estimate will heavily depend on the type of RAM that is used within your system, although for the purpose of this post I will use PNY RAM as a reference in-which sells for £45 from various retailers. RAM generally is the most deceptive to failures within any computer system however and due to this fact I will estimate at £10 second hand as a result).
Motherboard: ASUS

?? (I need more information in regards to the motherboard that is used. ASUS, as a motherboard manufacture, produces a wide variety of motherboard's both for professional and home user's which will effect the price point somewhat even as a hand-me-down estimate).
Sound Card: ASUS

?? (Once again the price will point heavily depend on the manufacture model, although for the purpose of this post I will use the Xonar DX/XD as a reference which sells for £50 at various retailers, my estimate would be £20 used).
HDD: 500gb Seagate (A standard Seagate HDD with the capacity of 500GB will sell for £30 from various retailer's, second hand I would estimate at £15 depending on condition i.e. worn arm est).
Case: Coolermaster haf 932 (Price new £90 from various retailers, second hand I would estimate at £50).
Before attempting to sell either-one of these mentioned component's I would try and perform a series of stress test in order to be rest-assured that everything functions correctly.
http://www.passmark.com/products/bit.htm