I purchased an Alienware system in 2003ish that died about 4 yrs ago. I just found what was claimed to be a new mobo if the same model on ebay and bought it. I installed it, along with a new PSU (bought when I thought that was the original problem). When plugged in I get a green light on the mobo (?), but upon powering up nothing happens. Not one peep, beep, or fan.
I would dance at someones wedding if they could help me. I don't have the bucks for a new system (no job). System is all original with exception of mobo (although same exact model as original) and 3 additional fans.
System: PSU 450 watt
Mobo: Asus P4T533-C
Memory: 512 MB RAMBUS
Graphics: Hercules 3D Prophet 9700 Pro
Well then, that sucks. It is either the motherboard, the PSU, or the CPU. Chances of it being the CPU are very slim. The PSU is running power to the board, as evident by the green light on the motherboard. So there is a chance it is the motherboard. Or the PSU. Eh.
Do you have any friends that are willing to let you borrow their computer to use as a test platform? You can put your PSU in their system, and if it boots up then its not the PSU.
On the flip side, you can also see if your friend will let you put your motherboard into his computer using his HDD, PSU, and GPU. Don't worry about drivers, this is just to see if it boots at all.
Also run a memory diagnostic, by removing all RAM and trying to turn on the computer. If nothing still nothing, put one stick into the first slot and try it again. If nothing, swap with another stick. If nothing, then RAM might be okay.
One thing that may have happened is that your old PSU went out, and when it went out it took your motherboard with it. If this is the case, you RAM may have been damaged as well as your GPU. While this is rare and there are safe guards to prevent this, it can still happen.
Its a crap shoot. Its possible the seller lied, or it worked when they tested it but it failed (what are the chances? Well, unfortunately high enough) during installation, during shipping, etc....
When you installed the motherboard, you took the proper precautions to discharge any static electricity on your body first, correct? Also, you didn't plug the PSU into the motherboard without first unplugging and discharging any power left in it first, right?
So many things could have happened, or the seller was a flake. Either or. But we'll figure this out, if you have a budget I'm sure I can find a legit retailer who has a replacement board cheap enough.