Spell scarcity is a concept that I think would greatly enhance any server, just like the lack of really powerful magic items enhances PoTM. A scroll of any spell becomes highly meaningful to wizards, and finding a cache of even low level scrolls (if they are currently unknown) would be the equivalent of finding the tomb of King Tut. Implementing scribe scroll would create a flourishing wizard-to-wizard market for new spells, as characters would potentially offer lopsided trades in order to fill out their spellbook repertoire.
Needless to say, it would still be possible for a cabal of wizards to conspire to each study different spells and then exchange them among themselves, but I am not sure that would be terribly satisfactory for the players involved -- each character would end up with identical spellbooks, which means they are all reduntant from a we-need-a-wizard-in-our-party perspective.
For rogues and bards and UMD, I think it is perfectly acceptable to have a few merchants about that deal in first and second level scrolls -- given the chance of failure with UMD, most rogues or bards are likely to be relying on a select handful of basic level spells anyway (mage armor, protection from alignment, magic weapon, ghostly visage, camouflage, etc.). I don't think you need or want any merchants that deal in scrolls beyond those levels. Besides, you want rogues and bards to be safeguarding their precious scroll loot just as much as wizards do, right?
But I agree with the foundational premise stated earlier -- you need to implement some sort of scroll scribing if you are going to eliminate merchants.