Like was said a couple of posts prior:
With a BLACKLISTED - N900T - you can:
1) Root, and Use Region Lock Away, it may possibly unlock the sim slot - so you can use an At&t based Sim Card. (Or, you may still need to get an unlock code from someone.)
2) Or - Sell for what its worth on Ebay.- Cut your losses, and find another Handset.
I have bought more than a few from people here locally (different models), seems Craigslist is running rampant (or rather the criminals) with those who are buying a phone, selling for cash on cl, and then reporting lost / stolen for an insurance claim. Then the buyer is stuck holding the bag, and has a tablet. (Or, bends and changes carriers possibly). Its sad but the only true way to be sure the device will be able to be used, is to go with the seller to (Tmo, Vzw, Att etc.) and get the rep to note the account. I have tried to do so via the 800 number, and had mixed results.