They're yours. Even if the mail was sent to a different address, the package was in your name, so yeah, they're fucked. I'd recommend going up to them and explaining the situation, but also tell them that it's illegal to open your mail, even if it's at a different address. When they ask how long, it's up to 5 years, plus $1000 if they're unlucky, so just give'em back.
Also, I'd leave 1 star for the ass that shipped the pair to the wrong address. He should be using the paypal address, and not your ebay address. He's a noob, and it's his fault for fucking up.
I put the STD in STUD, and all I need is U!