For a while I thought like the OP - I didn't believe any ad that said unfired, and "only 20 rounds down the pipe" was equally spurious sounding.
Then I bought a rifle, put 20 rounds through it, and then sold it because I decided to switch to shooting left handed. And now I have a rifle bought last May that I still haven't fired, life and Covid got in the way and I've only shot guns in hunting related circumstances since April of last year.
So if I can do it with my limited funds, I can totally see how others, many of whom have far more disposable income than I do, could also do this.
That said, I still take the claim with a grain of salt. If the only supporting evidence is because you said so, I'm buying based on the condition I can see in pictures not the claim its new/unfired.