It is perfectly legal to sell/purchase a pistol in AZ @ 18yrs old via private sale.
The only restriction is that an FFL cannot sell a pistol or pistol ammunition to anyone under 21.
When I was 18 I would normally just drive to multiple smaller shops looking for ammo and explain that I was buying the 9mm ammunition for my 9mm rifle (really miss my camp9). Which should be even easier now that PCCs are so much more popular.
It's cringey AF seeing people refuse to sell to anyone <21 via private sale when it's perfefctly legal.
It's almost as bad as wanting a buyer to meet them at an ffl for a bgc/transfer or demanding a copy of the DL for their bill of sale and "records".
I was buying ammo from a store when I was 10 years old.
My brothers and I would collect pop bottles along the highway and trade them in for ammo.
Of course that was a little while ago.