If none of the boot modifiers work (holding down other buttons while powering it up) then it's either bricked - probably due to a dodgy rooting attempt or custom firmware install, or possibly even a broken OTA update. There is no way to recover form this.
That does assume you've never got it to power up properly. I forget if you said you had or not.
If it's an intermittent fault (it powers up OK sometimes), then it's a hardware fault (hardware damage, water ingress, etc).
You have two choices:
1) It's probably under warranty still - most G1s are under a year old still. You could give HTC a call and see what your options are. Some people have had success with HTC simply swapping the handset for a brand new one, even though it was bought on eBay.
2) Return it to the seller.