Max Megablast

In order to purchase items from Max (you will need 600 total Luminary Research Notes to buy all three items), you need to first equip one of the Damaged Diving Helmets floating in the water around him. Once the helmet is equipped, you will be able to interact with three different types of luminary fish (Luminescent Grouper, Luminescent Eel, and Luminescent Jelly) in the zone, which will then follow you for 10 minutes each. The equipped helmet has an interactable button that turns the lamp on or off; I found it easier to spot the luminary fish with the lamp off. You can have all three fish following you at the same time. Bring the fish back to submarine above Max, swim into its beam of light, and you will get a large random number of the research notes deposited to your bag for as long as the fish timers are active and your light is on. (I had all three fish and got 868 notes for the time my fish were active the first time and 1,392 the second time.) Once the fish timers wear off, if you need more notes, go out and get more fish and bring them back again. Repeat as necessary. Important note: The Whale Shark will one shot you if you have your lamp on! Make sure to keep your eyes open and turn the lamp off as soon as he spawns. If he kills you, you'll need to get a new helmet and start over gathering new fish.

He seems to be currently bugged. You can click on him and he responds, but won't let you buy anything

For the Trading Post, he tends to move locations everytime you see him. I wasn't able to interact with him until he moved to just under the Submarine, and then i was able to buy stuff from him.