|I wouldn't put too much faith in what that person has to say and it's obvious he doesn't stand behind his work.|
The computer isn't going to post anything on the screen unless the ram is installed and I can't tell from what you've said if it is installed.
It's pretty hard to know if a motherboard is bad. It's kind of a last resort conclusion where if it's not this or that then the only thing left is the motherboard. And the only way you know for sure is to swap it with a known good motherboard and it that works then the other one must be bad.
They can go bad due to mishandling, static or a power supply going bad can also do it.
What you describe now--that it power up for a few seconds and then shuts off--could be the power supply, motherboard, cpu or a combination of those.
You're not really green until you're soylent green.