OP,
Make sure you do your homework. What you said (that the device is unlocked so you can use with any carrier) is not completely correct. Even though the Ipad Air comes with support for both cdma (Verizon, Sprint) and gsm (ATT, T-mobile), Sprint would not activate sim cards for unlocked Ipads, unless purchased directly from Apple. That might have changed, but read this article:
http://www.pcmag.com/article2/0,2817,2426795,00.asp
You might include this just in case the buyer uses Sprint, or you might get a return.