There are two inventory glitches with traders:

1. Keeping a consumable item after using it

2. Selling equipped items

3. Keeping a consumable item after using it
   Condition:
   This is actually not related to trading but merely most prevalent in that spot. You need a dialogue in which the hero says a line and gets no response from the NPC afterwards. For example at Ignaz "Show me your wares." works, but also telling Carl that you want to get stronger works. On the other hand Orlan's trading dialogue doesn't work since he always makes a reply back to the hero before showing his wares.
   Method:
   Start the dialogue, open the inventory, close it, open it again and use the consumable. The use of the consumable has to be timed so that the end of the hero's line interrupts the use at the correct time. For potions you have a very big window, for bags it's very small.

4. Selling equipped items
   Condition:
   This is similar to the above glitch, but obviously it has to be a trader this time.
   Method:
   Start the dialogue, open the inventory, close it, open it again. Now you should be able to menu onto equipped items and can sell them.
   Special case Cronos without the line: Sometimes the hero's "Let's deal." line doesn't play. In this case only do a single tap of the inventory key just as the animation of the dialogue choice window ends: Select "Let's deal.", press Enter, wait for the window animation to be almost done, tap the inventory key.
   Use:
   If you sell a scroll, you can now use it infinitely. But this effect will be gone after a save&load. This glitch can cause crashes. For fire rain first go through a loading zone to prevent crashes. The game will also crash, when the NPC dies and gets his inventory cleared. That is why the game crashes in Any%, if you sell the dragonsnapper scroll to Ignaz in chapter 1, kill him with the Ice Block glitch in chapter 3 and then linger around near him until the Ice Block effect runs out. Also you can stack the effects of rings, amulets and belts, but not armors. This effect will stay after save&load.
   You can also sell weapons, but there is no known use for it.