This Guide is now outdated, unless you plan to play on version 0.5.0. Though I don't see any reason to, since it has the same maps and ingredients as 1.0.X but misses all the other bug fixes improvements and customer requests.

It is a bug that was introduced with the game version 0.5.0.
People who suffer from this bug never see the dwarf or the wandering merchant, which effectlively prevents you from obtaining crystal ingredients.
If it's been a while since you last saw the merchants and are worried that you might have the bug don't worry. If you saw either merchant (dwarf or wandering) at least once during your save file you do not have the bug. Their appearance is random. Give it a few more days and they will show up again.

These guys are NPCs from the category of extra traders. Occasionally they should come by your store and they are always the first NPC you will see on the day they decided to visit you.
You can read more about NPC types here

They are important NPCs because only they are able to sell a certain kind of ingredients, namely the gems or crystals or whatever you want to call them. These gems are great because of their special ability: They teleport your potion to their paths end in contrast to normal ingredients that just move the potion along their path
The dwarf merchant speciallizes in gems. The Wandering merchant sells Herbs, Mushrooms, Gems and Enchanted Paper. Basically every consumable except for salts and reagents like nigredo.

Fahlgorithm on the Discord has made a mod that can fix broken save files. You can find it here[github.com]

1. Start a new playthrough and finish the tutorial on day 1
2. On day 2 finish the 1st chapter of the alchemists path before starting day 3.
3a. If you didn't see either of the 2 merchants you will have to start over from step 1.
3b. If you see either the wandering merchant or the dwarf merchant enjoy playing. Your savefile works correctly!

• Finishing Chapter 1 asap is very important. If you don't finish it the merchants will never come.
  
• DO NOT SAVE OR RELOAD THE GAME BEFORE DAY 3. Reloading seems to set the save files into the broken state. Only save once you do see the merchants. If you need to try again always click "New Game" in the main menu.
  
• It might still take you a few tries even if you follow this guide exactly. But the probability of this guide working seems to be very high from my and other peoples tests. More often than not one of the merchants did appear on day 3.
  
• Chapter 2 does not seem to influence whether the bug appears or not. So strict adherence to only doing chapter 1 goals isn't neccessary as long as you complete chapter 1.

Good luck.

Special Thanks to LotharLive on the Discord for helping me Test the effects of reloading and chapter 2 completion on the bug.