Buying phone on eBay/Craigslist ?

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http://forum.xda-developers.com – I have been looking to buy a Samsung Galaxy S3 (phone no contract). Someone is selling a brand-new, never used t-mobile galaxy 3. Can I called t-mobile and check whether the phone is legit (not stolen)? And if they say it is legit, can there be a problem down the line if the guy who originally bought the phone from tmobile cancels his contract and does not pay the cancellation fee? (HowTos)