I don't keep geckos so don't take the below as word. I was however intrigued with the difference between normal calcium and calcium with D3 so I browsed the interwebz.
According to a few places out there, d3 is added to calcium as it helps with the absorption of calcium in reptiles (depending on who you ask). There looks to be a general agreement out there that calcium with d3 is not a bad thing for any reptile and is actually of benefit to nocturnal animals that may not get their necessary UV requirements from a bulb setup particularly in the case of geckos. A few folks actually recommend a mix of standard calcium and calcium with d3 spread on alternate days (2 feeds standard and then 1 with d3) and this looks to be a popular choice with leopard gecko keepers.
As mentioned, don't take the above as word, its just my interpretation of it from browsing the net. I won't say the salesman is wrong because I don't know his background but for the same reason I can't say he is correct.
I hope those with experience though will share and help us both out.