I have a yellow nape, and three other amazons, and a moluccan cockatoo. Find out if this bird has been handled much, or left in the cage for years. Most birds are afraid of strangers, and will not let a stranger handle them, but talk to the bird, and see if it likes you before taking it home. My amazons only learned to talk when they were young, and did not learn much new after 2 and one half years old. Problem is, they often do not talk when strangers are around, so you will have to take someone elses word for it.
Amazons do have moods just like people do.
Sometimes they want to be handled, and other times they want to be left alone. You will learn the signals, and leave it alone when it is not in the mood to be handled. If you get the bird, go slow, and be patient. Do not force it to come to you until it feels comfortable with you. It is the best book out there on taming and training.