When buying items from Luna, buy bottom two items on the third page. You can only buy 6 of each at a time so you will need to eat them to buy new batch.
I figured it out after ask this question like 30mins later after the ghost drain my hp like hell and saw that girl stand there and didn't know what to do but i know now 👍
also i stumbled on a bug(?) In the college building, i keep getting teleport randomly on the 3rd floor when i reach the 3rd floor and also, it is possible to unlock scene that I've past the condition to unlock it like luna scenes after i bring wendy back
so I've done the quest that bring Wendy back and luna left but idk that there's a scene of mac x luna so that's seems to be unavailable to me now, that's the question