Most of the time the scammers will wait for the dispute period to end with paypal, which is 45 days, and then leave you high and dry.
1. Buy direct from t-mobile if you can.
2. If you are buying from a local person/craigslist, meet at a T-mobile store and ask an agent to check that it is free and clear with no outstanding payments due. Most of the time if the person is not trustworthy they will not meet you at T-mobile.
If the phone is blocked no amount of begging or pleading will get T-mobile to unblock it. It has been tried over and over and they will not allow you to pay any unpaid balance. The only one who can do it is the scammer who is long gone at that point.
History: Treo 600, Treo 650, Treo 700, HTC Titan, HTC Diamond, Nexus One, SGS1 (Captivate), Samsung Infuse, Galaxy Note, ATT Galaxy Note 2, T-Mobile Galaxy Note 3