iPhone "This device isn't eligible for the requested build"

Dear all,

I'm new to smartphones and recently purchased a 64Gb iPhone 4s on ebay (in hindsight a silly thing to have done).

After getting sick and tired of iTunes asking me to update the phone, yesterday I ran the update. After this had been done Apple locked the phone and iTunes told me I needed to "restore" it.

When I then tried to restore I got a message, "The iPhone could not be restored. This device isn't eligible for the requested build." The phone is therefore completely inoperable.

Is anybody able to let me know why I have this problem (I assume the phone isn't correctly registered or something) and if/how it can be rectified?

Any feedback greatly appreciated.

Many thanks,
DC

Thanks very much folks! The phone is working again now.