Minecraft – What type of blocks/items can’t you obtain by trading

Since the only items impossible to obtain in peaceful are mob drops (that are not obtained otherwise) and derivatives, here's a list of unobtainable items:

• Blaze rod
• Music discs (with the exception of music discs "13" and "cat")
• Ender pearls (even though you can obtain an eye of ender by trading)
• Spider eye
• Ghast tear
• Slimeballs
• Magma cream
• Nether star
• Wither skeleton skull

There are only a handful of direct derivatives:

• Brewing stand (requires Blaze Rod)
  • This leads to no potions as well, especially those that use a Fermented Spider Eye, Ghast Tear, Blaze Powder, or Magma Cream
• Sticky Piston (requires Slimeball)
• Beacon (requires Nether Star)
• The Wither (requires 3 Wither Skeleton Skulls, but you're not going to spawn that on peaceful anyway)
• Fire Charge (requires Blaze Powder)
  • Also limits available Fireworks

And here is the list of drops you can still obtain:

• Bone (can be found in temple and dungeon chests)
• Rotten flesh (before snapshot 12w22a could be traded with villagers but can also be found in temple and dungeon chests)
• Strings (obtained by: destroying cobwebs and tripwires and it has 53% chance of spawning in a dungeon chest)
• Arrows (can be crafted with other resources available in peaceful, can be found in temples and can be traded with villagers)
• Gunpowder (will be possible to loot in dungeon chests)
• Gold nuggets (can be obtained by placing a gold ingot into a crafting area)
• Dragon egg (reaching the end is possible by trading eyes of ender with villagers; apparently the ender dragon won't despawn on peaceful and the creation of a piston is still possible)
• Glowstone dust (can be obviously obtained in the nether and by trading with villagers)

It is also to note that a lot of items, that are possible to obtain, won't be renewable anymore, like:

• Bone (previously farmed by killing skeletons or wither skeletons)
• Coal (not charcoal, previously farmed by killing the wither skeleton)
• Gunpowder (previously farmed by killing creepers, ghasts or witches)
• Strings (previously farmed by killing spiders and cave spiders)

There are actually quite a few number of "compactable" blocks as of Minecraft 1.14.

For reference, some of these blocks are reversible, meaning that they can be turned back into 9 or 4 of the base material by placing them in a crafting grid or by breaking them.

Others are non-reversible, meaning they either cannot be turned back into their constituents at all, or else you receive less than 9 or 4.

Blocks crafted as 3×3 of base material

• Block of Coal (reversible)
• Block of Iron (reversible)
• Block of Gold (reversible)
• Block of Diamond (reversible)
• Lapis Lazuli Block (reversible)
• Block of Redstone (reversible)
• Block of Emerald (reversible)
• Hay Bales (block of Wheat, reversible)
• Melon (made of Melon Slices, not fully reversible unless broken with a Fortune-enchanted tool)
• Slime Block (reversible)
• Bone Block (reversible, made of Bone Meal)
• Packed Ice (not reversible, from Ice)
• Blue Ice (not reversible, from Packed Ice)
• Dried Kelp Block (reversible)
• Hay Bale (reversible, from Wheat)
• Nether Wart Block (reversible)
• Prismarine Bricks (not reversible, from Prismarine Shards)
• Purpur Block (not reversible, made from Popped Chorus Fruit)

Blocks crafted as 2×2 of base material

• Bricks (made of Bricks (item), not reversible)
• Nether Bricks (made of Nether Bricks (item), not reversible)
• Block of Clay (made of Clay (item), reversible when broken)
• Snow Block (made of Snowballs, reversible when broken with a shovel)
• Glowstone (made of Glowstone Dust, not fully reversible unless broken with a Fortune-enchanted tool)
• Sandstone (made of Sand, not reversible)
• Red Sandstone (made of Red Sand, not reversible)
• Wool (made of String, not reversible)
• Block of Quartz, (made of Nether Quartz, not reversible)
• End Stone Bricks, (not reversible)
• Magma Block, (made of Magma Cream, not reversible)
• Prismarine (not reversible, from Prismarine Shards)