Alright, I'm not one to rat about other people, but seeing as the tendency just keeps on going, I think it's about time that someone makes a PSA info post about this, so people can make an informed decision.
For a while there have been buyers in zone chat that will buy all your seemingly trash ingredients such as fleshfly larvae, spider eggs and cheap mushrooms, and they will usually offer the MM price or around there, which is rather low for these ingredients.
Seemingly a good trade for you, right? Thing is, you're basically selling away free profit for these people, as they will use the many thousands of ingredients to craft large quantities of poisons that they sell off for good profit to NPC vendors.
I believe that people have the right to make an informed choice and decision, and I think this is falling in the category of a dupe or scam, as essentially one party is privy to information that the other party has not got, meaning that what seems to one party to be a nice and good deal, is in fact a scam where you're being duped into selling away your profit. If you knew about it, you could just have used those ingredients yourself to make profit by selling the poisons to the NPC vendors.
Ultimately Zeni should fix this by making crafted poisons and potions have a 0 cost or 1 cost per item instead of the current value. After all, people craft poisons and potions to use them not to sell them th vendors.
Anyway, feel free to sell off your larvae and eggs and whatever to these people, at least now you have the relevent information to make a deliberate choice about it.