When you say unsealed I think you mean that there were no seals holding the box or bag closed? Rather than the motherboard not being in the bag?
If so, yes, this is typical of modern motherboard packaging for most SKUs out there. There are no seals on the box or the anti static bag inside the box. The only things that tend to be sealed in bags in those boxes are pack-in stuff like SATA cables for example. I think you're good to go.
Now if the board wasn't in the bag, that would be bad.
Please replace your power supply. You've been lucky to get a decade out of it. It is old and tired and it is a risk to the rest of your components. Purchase a new, efficient, reliable power supply and protect your investment in those new parts.
I ran a Corsair HX850 from 2010 to 2016 and when I replaced my entire machine I said "It's time for you to retire" and that was that. I sold it to somebody who was more of a risk taker than me, and got a new unit.
10 years is a long time for a power supply. In one machine I built (on a budget), a Corsair CX600 V1 died after 3 years, on another an EVGA 500B (I know, I know. I sinned.) died after 3 years as well. And it died the dangerous way, throwing out wacky voltages that were way out of spec rather than actually dying. Thank goodness I caught it and replaced it.
PS
Please use a surge suppressor such as a power strip that has this feature. It's a good idea to have another surge suppressor in your electrical panel in your home. If your surge suppressor works with CAT5/5E/6 etc as well as coaxial that's a bonus. Surges can come along your ethernet cable.
Great PSU options:
https://www.overclockers.co.uk/evga...plus-gold-modular-power-supply-ca-03j-ea.html
EVGA SuperNova G3 850W (if you want to SLI maybe someday?)
Or a more affordable 650W model:
Superflower Leadex 2 650W
https://www.overclockers.co.uk/supe...old-modular-power-supply-black-ca-05b-sf.html
These are both very good PSUs
