The two methods that come to mind are to sort your loot after each hunt using a backpack system in one depot. You can have a blue backpack for Blue Djinn, green backpack for Green Djinn, etc. You can fill up these backpacks with others full of loot and you will now know where all of your loot is for a particular NPC when you need to sell it.
The other option is to do a similar system but based on depot box. After every hunt drop your Blue Djinn loot in depot box II, then all Green Djinn loot in depot box III, etc. This is quicker than the backpack method for storing loot as you can just throw it directly in the depot box without having to worry about nesting backpacks. It may be a bit harder to remember which box is for which NPC at the start though. Also, it may be a little more time consuming later if you need to pull all of the loot out into backpacks to sell or send them.