If it's very picky abvout what PSU it will post with, it's also possible that the VRMs are going/gone on the motherboard which causes it to fail on a power supply who's 5V or 12V lines aren't within a certain range. It doesn't even have to be the middle of ATX spec, it could be on the high or low end but still within spec.
Does the ebay PSU power a different PC without any problems? If so then you're probably looking at something other than the PSU as the culprit.
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