RazerZ :
You could stress test the CPU and GPU with programs such as Prime 95 and Furmark. For the ram use memtest86.
You could also use Speccy to help verify the system listed.
Be careful with buying used systems... many tend to be overpriced and have junk parts. If you insist going that route post the build here before you take a look at it.
This is the build.
CPU: Intel Core i7-3820 3.6GHz Quad-Core Processor
CPU Cooler: Corsair H80 92.0 CFM Liquid CPU Cooler
Motherboard: Asus Sabertooth X79 ATX LGA2011 Motherboard
Memory: G.Skill Ripjaws X Series 16GB (4 x 4GB) DDR3-1600 Memory
Storage 1: Samsung 830 Series 128GB 2.5" Solid State Drive
Storage 2: Western Digital Caviar Black 1TB 3.5" 7200RPM Internal Hard Drive
Video Card: EVGA GeForce GTX 690 4GB Video Card
Case: Fractal Design Define XL Titanium Grey ATX Full Tower Case
Power Supply: Corsair Professional Gold 850W 80+ Gold Certified Fully-Modular ATX Power Supply
Optical Drive: LG GH24NS90 DVD/CD Writer
Fan: Cougar Vortex 120 X 2 (replacing H80 stock fans)
Fan: Cougar Vortex 140 X 3 (case fans)