Im wanting to sell my iPhone3Gs so i can buy myself out of my contract
and buy the new iPhone4 .. Ive had a look round Ebay but the differance in prices are Huge .. Starting from £300 to £700 .. What do u think is a
Reasonable price for a Used 16Gb iPhone 3Gs ..?

Factors that impact price.

Old bootrom
Officially carrier unlocked
Unlockable baseband 05.11.07 or lower
Is it in 'as new' shape?

If it is locked to a carrier - expect to sell it for less. If it can't be unofficially unlocked you can't expect to get very much for it.

Also consider that the 3GS is now available at subsidized price of US$99 this reduces the resale value of current 3GS models

Originally Posted by Olethros

Factors that impact price.

Old bootrom
Officially carrier unlocked
Unlockable baseband 05.11.07 or lower
Is it in 'as new' shape?

If it is locked to a carrier - expect to sell it for less. If it can't be unofficially unlocked you can't expect to get very much for it.

Also consider that the 3GS is now available at subsidized price of US$99 this reduces the resale value of current 3GS models

for a mod you shouldnt put u can buy them for $99. You know that to get it at that price you have to pay a overtal total price of $1k+ over 2 years.

How much is it used? What does the outside look like? Does it look new? What is the screen like? is it scratched up, or was it protected with a screen protector? The look make up an incredibly huge amount of difference, because that is what really makes it look new, especially for the iPhones glass screen where practically every scratch is seen. What also makes a difference is wether it has the newest firmware or not, and wether it is jailbroken or not. My suggestion: latest firmware and untethered jailbreak, however don't install anything other than cydia. If you have it unlocked, you can also advertise that...
It would be best to post a pic of your iPhone, so that I could see about how much I would pay, if it had everything software-wise mentioned above.
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Is iOS 4 out yet? If yes, I would wait for iOS 4 jailbreak before selling. For software updates, the iPhone Dev team is usually pretty quick with jailbreak.
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UPDATE: Redsn0w available for iOS 4!