How to prevent Bad ESN

You can check on T-Mobile's own checker online to see if it's blacklisted:

https://www.t-mobile.com/verifyIMEI.aspx

----------

But if they decide to report it stolen afterwards, nothing you can do about that unfortunately. It's a risk you'll have to take with Craigslist.

 

Even if they report it stolen you would still be able to use it on tmobile.

 

If you're buying it in person, meet the seller at a carrier store so that you can activate it with them there to make sure that it works fine. When I sell phones I always meet people there to avoid any problems. I always ask for the money first though then walk in the store with them.

 

eBay generally has less risk because of the seller reputation. You can always dispute with Paypal if you have an issue with an eBay phone.