Seasonic SS-600HT 600W PSU Power Supply not working

I bought the above off somebody on eBay and the online community has told me this is a good PSU. But it hasn't been working with 3 of my motherboards and 2 of my cases. For the Pentium 4 motherboard the CPU fan keeps spinning but nothing happens. For the Core 2 motherboards (asusp5q se2 and asrock wolfdale d667 (what looks like a G31m)) the computer stays on for a second or two then goes off. It doesn't do that again unless I unplug the PSU from the motherboard. This is in a Jeantech Unicorn Case. The P4 chip case is unknown.

What's the problem with the PSU? I tried 2 different PSUs of 215 and 300W and they worked fine. Is it the types of components I put in. I have a Q6600 and am waiting on a vapour x 5850. Does the PSU need to be under some kind of stress before it works properly? It's currently under ~100W probably. These are the features:

· High capacity output (continuous)
· Dual +12V outputs
· Multiple 12V connectors (4-pin ATX, 6-pin PCI Express, 8-pin EPS)
· High efficiency operation (up to 80%)
· Active Power Factor Correction (up to 99% PF)
· Smart and Silent fan control (S2FC)
· 120mm Cyclone cooling fan (Double ball bearing fan)
· Four serial ATA connectors
· Universal AC input (100-240V Full Range)

Could any of these cause a known problem?

your case has nothing to do with how the power supply works, all it can to is short out the power jumpers on the motherboard. if you get any sign of power that is working fine.

if it doesnt work on 3 different systems its dead. probabily the cheap electrolitic capacitors they put inside. chances are it was dead when the guy you bought it from shipped it. i bet you have been had.
there are ways to fix it but im not getting into that with someone that doesnt know electronics. you could fry your computer or burn down your house if you mess it up.
you need to get a new power supply, seasonic isnt quality, try thermaltake, Pc power and cooling, antec, cooler master, corsair or rosewill.
and buy new, its the most important part of your system, dont skimp plus its returnable when new, only a few dollars more.

Wow, you put your noob out there for everyone to see.
No idea that Seasonic provides the internals for most of the highest rated psu's from PCP&C , Corsair and Antec ?
Rosewill and quality, in the same sentence ? They have shown a little improvement but that's about the extent of it.
Should have just stuck with "you've been had".

(1) as you found out, DO NOT buy used PSUs along with MBs GPUs and memory.
(2) Use google to check the reviews of a given PSU.
Ex: http://www.hardwaresecrets.com/art [...] view/936/9

Sometimes you have to look a little closer at the review conclusions;
For example this review knocked the OCZ PSU down for what they said was High voltage outputs, I looked back and To Me thier conclusion on Voltages are BS!!
Ex: http://tech-reviews.co.uk/reviews/ [...] 00-watt/6/

It depends on how the bios is configured, Normally if you have a dedicated graphics card you should plug your monitor into that one. And the bios should be set tothe pci slot instead of the on board igp