IPhone 4s AT&T Help No Service?

Hi, I recently sold my old iphone 4s on ebay. I send to the seller and when he receives it, he says that his sim card and his friends are not working, and there's no service. The phone is not stolen, I was the only owner. I reset it to factory settings and took out my sim card before shipping. Please if anyone has any advice or how to fix this, please let me know.

Maybe it is locked to your carrier?