buying iphone from ebay and activate it?

[epiphone93]

I want to buy a used iphone 4 off of ebay but I'm a little confused about how to go about activating it. I currently have a iphone 3g on AT&T. Can I just buy the used iphone 4 from ebay and take it right to my AT&T store and get it activated or is there more too it then that?

Thanks.

 

[Anthony]

its simple:
Get the iphone 4
take the simcard out of the iphone 3g
then put it in the iphone 4.
Simple as that,