I think the seller is full of shit. He may never have fired it, but every Glock you buy comes with a casing in that caliber from the test fire... so it's been fired once.
Side note: I don't trust my abilities and familiarity with any of my weapons brand new until I've put a few 100 rounds through them... and then I strip and clean them. I didn't carry my 1911 until I put 250 rounds through it and gave it it's first cleaning.