I've had good luck with Craig's List but remember to follow the rules:
1. face to face ONLY and
2. cash ONLY!
You can always meet at the carrier's store and get the phone activated right there, that eliminates any possibility of a bad ESN or...
A little Craigslist tip, send your seller multiple extremely low offers from separate email accounts, lower his confidence a little before going in for the real but still below value offer.
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