Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
The sim card contains information about your account with your carrier. You should remove it unless you want to pay the bill for the person you sold it too.
 
The sim card contains information about your account with your carrier. You should remove it unless you want to pay the bill for the person you sold it too.

If the sim card has been deactivated, it cannot be reactivated. It doesn't store contacts, so if its deactivated, it will only bypass connect to iTunes screen if the phone gets restored/updated.
 
Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_0 like Mac OS X; en-us) AppleWebKit/532.9 (KHTML, like Gecko) Version/4.0.5 Mobile/8A293 Safari/6531.22.7)

Take it out of the phone. It would be of no use to the person buying. He/she will be able to get a free one by calling their phone company, or they will probably put their own SIM in it.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.