Default IMEI added to blacklist by Tmoible

I bought my LG G2 exactly 30 days ago (11/23/13) from this local store , they advertised it as brand new on Craigslist, I paid $450, about the same price as if I buy from Ebay, since it is local store with good Google reviews, so I went with them.

The phone works very well, really love the phone. After exactly 30 days, this afternoon the phone stopped working, it won't register to Tmobile network. I rebooted it a couple times, still won't work. After I called Tmobile, they asked me where I bought it, they said the previous owner stopped paying Tmobile , so the IMEI was being blocked.

I pulled out the receipt, it has a 30 days warranty on parts and labor. I called the shop, they closed for the holiday until Friday

Did anyone else have similar experience? Am I scammed by this shop? What shall I do if the shop refuse to cooperate?

Thanks for your suggestions.

Sent from my Nexus 7 using xda app-developers app