Q: como puedo verificar si un codigo de itunes este activa y luego venderlo

Daniel,

If it is a phone you are selling, see some of these threads

how do I delete everything from my old I phone 5 but keep all of it on my new I phone 5. If i delete things off one phon…

deactivate iPhone before sale?

How ca I delete all data from my iphone permanently?

Also

Deauthorize your computer using iTunes - Apple Support

and see within there

Do I need to deauthorize my iPhone, iPad, or iPod touch?

You don't need to deauthorize your iPhone, iPad, or iPod touch, but you do need to disassociate your iOS devices from your Apple ID. Your iOS devices don’t use your available authorizations.