Pretty sure the first comic book shop I was aware of was essentially the back room of a head-shop in New Cumberland, PA (ca. 1975 - 1976).
My collecting pal Steve and I were only 10 or 11 at the time, but we somehow managed to talk my older sister into driving us across town. Among other books, I bought a spine-rolled ASM 28 there for pocket change, read it on the ride home, and it was instantly both the coolest and oldest comic book I had ever owned.
The thing I remember most is that the comics were stored, un-bagged, in fruit crates (they sold fresh fruit up front--I distinctly recall cantaloupe!--in addition to bongs, incense, "High Times", etc.), and that there was some sort of oddball, "flat-rate" pricing system based on issue numbers, with the early numbers of any given series being more expensive than the later numbers. It was a long time ago, but I don't remember grade being a factor at all...