What do I need to do to sell my iPhone 3GS? I've upgraded my iPhone 3GS to iPhone 4 (and my wife's as well). The new phones came with their own US/AT&amp;T microSIM cards and have been activated. So we still have our old phones with the old SIM cards in them. I've deleted the data off of the old phones, and I would like to sell them.

What do I need to do to sell the old phones? Do I remove the SIM from the phone and expect the buyer to put their own SIM in there? How do they go about getting a legit SIM from AT&amp;T to put in the phone? Or do they use the SIM that is in there? How can I be assured the old phones cannot be linked to my current account in any way?
One easy option, if you can use the store credit, is to sell them to Radio Shack for $200 [1].

If you prefer cold hard cash, or think you can do better than $200, there's a market for them on eBay, and it's easy to search for the going rate.  The phones will be easier to sell for more if you can jailbreak and/or unlock them in advance.  (How to do that is best left for another question.)

As for how a conscientious buyer activates your old phone through the proper channels, it's my understanding that once you've upgraded your line, your old phone can simply be brought into any AT&amp;T store and associated with a new account.  I.e., you can &amp; should sell the phone without a SIM.

Once it's been wiped and no longer has your SIM, the phone is no longer linked to you.

[1] http://reviews.cnet.com/8301-19512_7-20008186-233.html