Buy iPhone 4 to sell on eBay

I'm trying to buy an iPhone 4 to sell on eBay but I need to figure out how. I'm going to buy one for me and then, one to sell on eBay on my moms account. She's eligible for an upgrade and she currently has an iPhone 3G. (Which she only uses for texting, calling and playing words with friends) So what I want to do is upgrade her to an iPhone 4, yet since this phone has a Micro SIM, how can I make it so she can still use her old phone? I was thinking of telling the people at AT&T that she wants to give it to my dad, and then get a new sim card for to use in the 3G her but idk if they will agree to that. Have any ideas?

If you're due for an upgrade, I'm sure they will just give you the phone for you to do whatever with it.

order it online

- order the upgraded iPhone 4
- activate on your mom's account (which will disable her 3G sim)
- take 3G to AT&T store
- say you want to put the 3G on your mom's line (and take the iPhone 4 off - doesn't matter why, nor should it)
- they will give her a new regular sim for her 3G (should not be charged because you are just changing phones, not buying a new sim card. i have done this already and have not been charged)
- sell the iPhone 4 on eBay, etc.