The Church of the Lord of Dawn had plenty of magical items that could harm the undead.
But the Dawn Paper Edna had used to make the handbook was simply made from materials soaked, at the start of production, in water drawn from the holy pool where the Dawnstone was enshrined.
The water from that pool had to flow away somewhere. Rather than let the creatures near Dawn Tower benefit from it, the church might as well use it. Lathander would not mind them putting it to use.
The Church of Dawn had originally made the paper just to show off its affinity for light. They had only discovered later that undead could not touch it and that only truths could be written on it.
Yet it could serve neither as contract paper nor as a Priest's divine spellbook. In practice, only the church's upper ranks used it, mostly for internal correspondence. Lathander's high priests took after him, and the paper gave church members one way to work together with peace of mind.
As for selling it, how could they put something their priests used regularly into circulation? Lathander's priests cared about appearances. At most, they would use it to communicate with nobles who had infringed on the Church of Dawn's interests, making their warning unmistakable.
Otherwise, they left it alone. There was no need to bully the nobles that badly. They might not care whom they inconvenienced, but they had no intention of making enemies of the whole world.
The paper was not so precious that no one could buy it, though. Senior members of friendly groups like the Asking Sword Family, who could obtain the Supreme God's emblem, could purchase it—especially Young Master, who had spoken directly with their Supreme God.
But the paper was useless to the