You got screwed. The seller wanted more money and
this other person paid fast, yours however was
still lingering in the mail or on their desk.
Anyway you look at it, you have little recourse.
You could've cancelled the USPS MO, but then you
lose money. In all, the seller f'd you... but then
again, when it comes to reselling it usually comes
down to money. No surprise, sorry you experienced
this. Live and learn.