An item maniac is a person who will purchase certain items from the player at a higher price than what they would be sold for, like in 5th generation games. I own several valuable items and I want to sell them at the best price.
The description for the mushrooms suggests it:
It's popular with a certain class of collectors and sought out by them.
Is there an item maniac in Pokemon Sun/Moon?
Best Answer
Based on the Bulbapedia Item Maniac entry, it looks like these were only featured in Gen 5 (Unova) and have not been featured since.
Additionally, in this thread, people have denied their existence.
Another telling sign that these are absent in Sun and Moon is the fact that, while there is a Serebii page for the B/W item collectors, there is not a page for them for S/M.