Can I use Post-it’s As book markers on Shabbat?
Yes, this is permissible. There are those who are stringent unless it will be removed from it’s place within a day, and is removed from the pad of notes before Shabbos.
See Shu”T Minchas Asher Vol. 2:39, Nishmas Shabbos Vol. 7, pg. 206, Igros Moshe O:C 2:84 [s”v vehinei]. cf. Shmiras Shabbos Khilchisa 28:8.