[Yo-Yo]

The 21st Amendment to the Constitution sort of made buying liquor pursuant to State Law a Constitutional right.

Oregon state law doesn't prohibit 18-20 year olds from purchasing long guns.

Oregon state law does prohibit 18-20 year olds from purchasing alcohol.

So getting back to your original question "What would happen if he tried this against a Liquor Store?", the two cases aren't the same.

In one, the seller is following state law, and in the other the seller is not.

[ml/nj]

Oregon state law does prohibit 18-20 year olds from purchasing alcohol.

Actually that law was passed under duress, if I am not mistaken, and might reasonably be considered invalid.

ML/NJ