So what will a 3G be worth?

[illiniry]

No...it has to be unlocked to be considered unlocked.

Locked, used phones still go for around $300, which I don't understand at all. Can I sell it without the SIM card though, so I can keep it for the new phone?

 

[1appleAday]

no, you have to jailbreak it and then unlock it

So is it true that once the phone is jailbroken and unlock, the seller can take out the original sims and just sell the phone?