If the market Class is large enough for potions and scrolls to be automatically available is the assumption that one of each kind is automatically available? So there would, for instance, be one scroll of each 1st level spell available instead of a single scroll?
That is correct. You can, of course, decide that some spells are rare or unavailable. I typically do that with certain spells that I imagine as being "forbidden" and whatnot. So Animate Dead or Cause Disease scrolls might be rarer than Cure Disease.
In my campaign I use a rule for spell services avialaiblity that might be useful here.
Each urban settlement is dedicated to a particular deity (except Dwarven ones, naturally) and divine spell availability is normal for that market class for spells cast by a cleirc of that deity. If you want a spell cast by a different deity of the same pantheon (Emperyan vs Cthonic), you treat the market as if it were one class smaller. If you want a spell cast by the opposing pantheon, you treat the market as 2 classes smaller.
that's a solid house rule.
I also grant a couple of settlements improved market class for divine spells - those ruled by a divine casting class.