Okay, your thread title/opening post is a little confusing, so let's clear that up first. Slick Guns is a pretty neat search engine/database/service that posts up LINKS to deals. As far as I know, Slick Guns is basically an enhanced search tool and they don't sell guns. I'm not sure if they sell anything. You linked directly to an offering from iAmmo -- who definitely does deal in used and re-import Smith & Wesson pistols.
I own a 915 but it's NIB/mint and tucked away. These pistols are obviously older guns, much used, with varying degrees of wear (some so heavy you'll do a double-take.) I do have one buying experience with iAmmo on a used, formerly issued re-import 39-2. My review is mixed.
As that gun and this gun are different and (likely?) came from different places, I'm not sure how much my experience helps but I will say this:
Be prepared to receive a handgun that is below your expectations when it comes to finish and exterior wear. Without being able to pick it up before you buy -- it is a very tough game to think or hope that your "A, B, C, and D" is similar to what the seller is grading.
This scale of grading is no standardized industry scale. It's some guy looking at a crate full of old guns that often showed up on a cargo ship.
If you want to hear more about my purchase from iAmmo than that, send me a PM. For a gun that was heavily produced in great volume such as the 915 (very good pistol!), I would shop somewhere else. And while you do take a similar risk when shopping Gunbroker -- you typically get multiple pictures of your specific gun before you even bid. 915's are plentiful on Gunbroker and literally listed all the time. The supply is not short.