Does increasing your Insight unlock new items to buy from the Insight Bath Messenger?

Solution 1:

No. Insight does not influence stock at all, it only unlocks the shop when you reach 10 ( 1 in latest version) Insight and lose it goes under.

If you are looking to unlock stock you get all of it by:

• Progressing in game
• Finishing Chalice Dungeon
• Killing few specific hunters

Slightly off topic lore side:

Insight has tiny effect on game difficulty and is mostly there to keep the player immersed into game's lore. According to the game's lore insight is gained by learning about the world and coming into contact with the Great Ones (basically seeing what you shouldn't see) and allows to perceive more things. It's sort of interesting how some enemies appear and other enemies start using new attacks, that they might have been there, but none of it hurt you because you couldn't perceive them. Also, too much insight enables perception beyond what people can handle therefore driving them insane and there are references scattered throughout the game.

At any rate, if you are interested in lore I am sure someone has analyzed and written about it.

Solution 2:

No. The stuff the Insight Bath Messenger sells does not change regardless of your insight level. The only limiting factor of the Messenger are as follows:

• You must have 10 insight to unlock it

and

• The armor sold depends on what NPC hunters you have already killed

Sources:

• Experience
• Bloodborne Wiki