I bought my Galaxy s4 in August and it was working fine until now where he decides to stop paying his bill. Now T mobile blacklisted it. I have his phone number and address I called him and he said he doesn't know what I am talking about. What are my options? Can I change the IMEI or will the police be of assistance?
I would call TMobile and explain what happened and ask if you can pay off the balance owed on the phone.
Your option against him is small claims court. The problem is the expense of winning and what you would have to spend to collect any judgment you win.
This is a civil matter and the police won't help. You have a breach of contract claim, not a theft claim.
Changing the IMEI is illegal so that isn't an option.
If you do decide to go to small claims court, the first fee you will have to pay is the filing fee. Then you have to pay to have him served with notice of the case you filed against him.
Now once the case is filed, you will need evidence from TMobile showing the phone has been blacklisted unless the guy you bought it from is willing to admit that. TMobile has the right to charge you for reasonable copying fees for any documents you request from them to show that the phone is blacklisted. And TMobile won't turn over to you any documents on his account without the court issuing a subpoena if you are representing yourself.
And once you win, then you have to go about collecting your judgment, which of course has expenses associated with it.
You should never buy a phone from Craig's list again.