Before doing more, I'd try pressing the PMU reset button. It's a small button near the CD cable. Then try the power button.

If that doesn't do it, then most likely a bunch of capacitors have failed in the power supply. Odds are that if you opened it up, you'd find a number of bulging caps.

I've been given a few MDD after their power supply has failed. Just recently I went to power up an early MDD only to discover that the power supply had expired.

If the eBay power supply is a pull then it can fail tomorrow. On the net you can find companies that'll rebuild your power supply. Another option is a ATX conversion