Seriously, give it up. You are wrong. You can put an ATT SIM from another phone into the iPhone without jailbreaking. I've done it, my brother has done it, and I have a couple of friends with ATT contracts who bought secondhand (locked) iPhones who just swapped in their SIMs. I even bought an ATT GoPhone for temporary use and pulled its SIM and put it in my iPhone. No problems.
Yes, eventually ATT will notice and try to switch you to an iPhone plan but the fact remains that one ATT SIM is the same as any other and they're interchangeable.
You do not need to jailbreak to sell your iPhone if your buyer uses ATT.
Before you ask, no I cannot provide screenshots, video, documentation, etc. and I'm not sure you'd accept it if I could. Several others besides myself have tried to tell you that you're mistaken. You might consider re-checking your facts.
http://tnkgrl.wordpress.com/2008/07/11/the-iphone-3g-works-with-any-att-sim/
i know im right... ive tried it and it doesnt work
i provided evidence to support that... you have not