replace motherboard in manufacturer's system

I think my motherboard in my emachine is dead. I have a amd 2800+ system. When I plug in the power cable into the psu, the mobo would power on (everything would be running) without me pushing the power button on the front of the case. I know this because I replaced the psu already and also the video output won't output any video/image on the monitor. So the question is - can I replace my mobo with a used one on ebay? I'm just concerned because it's from an emachine or else I would have already done it since I've heard a manufacturer's pc has proprietary parts so it's not user serviceable.

Not too long ago there was a thread with the same question and I believe the consensus was that you'd have to replace it with the identical board as well as it would probably make more sense to build a new machine because of the likely cost. Sorry, don't remember the precise thread