Hi i just bought a macbook 1.8 core duo from ebay.. it sounds like a battery issue to me but maybe someone who knows mac's more can confirm. here is what is posted. .
"The computer boots up but the screen goes dim, then goes into sleep mode. You can wake it up but it remains dim. I believe the battery is done as i was haveing draining issues when it worked. I pulled the HD and put it in another macbook of mine and erased all the data and installed a fresh copy of OSX leapord to see if that would help. It did not."