Federal law imposes age restrictions on transactions involving ammunition.
State and local law may impose further restrictions.
For example,
some states require buyers to provide a copy of their state firearm owner card to the seller.
Other states might limit the caliber and type of ammunition that can be sold.
It is your responsibility
to know and follow the law in your jurisdiction.
Additional resources
Some regulations by state
Some state-specific regulations (this is not a complete list):