It may be a "Factory Refurbished" Glock. Basically Glock takes a used pistol, replaces anything that needs to be replaced and may refinish the slide, then sells that gun as "Factory Refurbished". It should have a gray tag wrapped around the trigger guard that says "Glock Factory Refurbished" as you'd see in this picture: http://gastatic.com/UserImages/87564/997405626/wm_1450382.jpg
Basically it's one step up from a used gun.
It may also be the case that it's in fact a used gun, never refurbished by Glock, and the dealer is just saying what he needs to in order to sell it. If it has aftermarket parts like an aftermarket barrel or magazine extensions, it's a used gun that was never refurbished by Glock.
I'd rather just buy a new Glock.