I've never thought about used custom built computers. I should look into that when I next consider buying a new system.
Overall that price is way high. I'm not sure, but I think that the warranty has a large role in used computers. No/short warranties should drive the price down, and the fact that it's used normally gets you to about 70% it's original price.
I'm having an issue with going into sleep mode, and I believe it's my processor. On fairly CPU intensive program such as left 4 dead 2, it goes into sleep mode fairly often, but on a dual threaded arma 2, there is no going into sleep mode. On something like Modern Warfare 2, my computer started seizing up when I was hosting before going into sleep mode. Could this be a cpu problem?