Laptop only boots when you do a hard SMC reset

From what the eBay seller of the system tells me:

Laptop only boots when you do this:

• Unplug power
• Hold power button down
• Connect reconnect power

After a few seconds it will chime, then let go of power button

Note it will run with fans at 100% the whole time. Also battery is dead, so if you unplug power, it will shut down. I found this tip on here on iFixIt.

Not sure what is wrong with the laptop, had previously thought it was dead and unable to boot, but it can when you do the above. I have tried SMC and PRAM resets to no avail.