Craigslist or Kijiji. If you are going to use Speakou, don't waste your money on an iPhone 4. A 3GS will do everything a 4 will do at about half the cost.
Yes, the iPhone 3gs will do everything that the 4 will do on Speakout, but it's also half the phone. The 4 has a much faster processor, better display, front and back cameras….
I would say Kijiji is your best bet for a used phone. I have bought and sold many blackberry and iphones on kijiji.
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