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Shouldn't be an issue. The buyer probably want to check on apple site to see what kind of support is left for it. You can also tell the manufacture date from the serial. If it's a 3GS, he/she probably wants to know if it was manufactured after or before the new boot-roms were used for jailbreaking purposes.
 
don't do it

Don't do it, it's a scam. I had someone try the same thing with me when I tried to sell my 3g on ebay. It sounded shady so I googled it and found that a legit serial # can somehow be transferred to a stolen iphone in order to activate it. I don't know the technical stuff on how it works, but it is a scam. Within an hour ebay contacted me because I had responded to the persons email, saying that any correspondence with his account was suspect because the account had been compromised.
 
So it seems like everyone agrees that selling an iPhone on ebay is a terrible idea. It's scam after scam. I'd say try to sell locally through craigslist or word of mouth even if you have to get a little less for it. It's not worth dealing with all the scumbags on ebay.
 
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