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I sold two gen 1 iPhones on eBay several weeks ago. They were locked and used but in good shape, and both had v2.0 software. I listed them for $350 buy it now, and both sold in less than 12 hours. I think I could have gotten more, but didn't want to screw around with it.
 
I sold my 8gb last week on Craigslist in Phoenix for $450 jailbroke/unlocked. I had a lot of people inquire about it too. Sold it in two days. I had it replaced at the Apple Store due to some pixels going out, so it was in New condition when I sold it. Beats ebay since I didn't have to pay shipping or ebay fees.
 
sold mine locally on craigslist on 9/2/08 for $400 within, literally, 5 hours. received nearly 20 calls. had replaced it prior to selling, locked.

i'd say, don't sell it for less than $400. good luck.
 
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