Well, the Z7S in its new form definitely is reliable as a rock. I have been crunching at FSB 430 for months now, current uptime is something like 1 month 2 days.
You obviously got a mobo with defective 4pin connector, that's the only reason I know it won't boot with 2 CPUs.
I'd try to RMA with Asus, just say the previous owner didn't give you a receipt, but the mobo must be within warranty because it's not even 6 months old.

Or if you know anyone with good electronic/soldering skills, maybe you can let him replace the faulty connector.

And don't buy mobos on ebay...