I might be wrong but I think I remember these scrolls are hard to find and are remains of past civilisation. Some people can decipher them, but no one can make them anymore.
GS found only 2 in all his years of goblin slay, when he visited lots of ancient ruins.
As other adventurers do not mess with goblins, it's possible they do not visit ruins and have even less chance to find scrolls.
These scrolls are extremely rare and their price is high for that reason.

GS had a pretty clever idea, for at least 2 reasons :
He wanted to kill that boss, probably because it was too goblin friendly.
The scroll probably can't teleport more than one person.