Lets roll this back a bit and make sure you have a Real Seller of a Real Parrot and not just a Scammer or Black Marketer!
First: Today's Date is 1, September 2017, which is the start of the ninth month of 2017. You are being told that this Blue and Gold is five (5) months old, correct? That being all correct, your Parrot was born long after 1, January 2017
Second: A Blue and Gold Macaw is covered under CITES II Classification and therefore requires a specific set of Paperwork for the 'Breeder' to sell this Parrot to a Reseller or to you! A Birth Certificate if you will. This requirement went into effect 1, January 2017, which because of the stated age of this B&G MAC requires that those papers exist and that you must get them as part of the sale including a Receipt of Sale.
Third: If this is in fact a Breeder, they haven't a clue what they are doing, since the Price of a MAC is greatly reduced with the loss of 'all' of its tail feathers. For that reason, I do not believe you are communicating with a Breeder. For the same reason, Resellers are faced with the same realities and take great care not to have a MAC in a cage that would cause that level of damage to a MAC's tail feathers. That leaves it to a stupid Owner or a flipper /scammer.
What to do:
Unless this individual has CITES II Paperwork for this Parrot - RUN!!! This does not sound like a proper (legal) sale to me! But hey, that's me. Its your choice!!! May You Choose Wisely!