[mothered]

They claimed that the box was unopened

What evidence did they provide In support of their claim?

I believe someone must have swapped the PSUs and returned it using sealed box method / sold it to them after putting a different model PSU in the box?

You've been SEd.

Could I return the PSU, as well as claim that my GPU was fried and request compensation?

Sift through their terms & conditions and see If there's any mention of compensatory damages.