This chapter is a complete guide for Demand and Supply errand. Here you can find tips that will help you to finish this quest in the most beneficial way.
Activating the quest
You can start this quest by speaking with two merchants arguing with each other in Meridian Village. After a short dialog they give you an offer - they can pay you for particular parts of a machine. They want two Lenses that you can get by killing Longleg and Snapmaw.
You don't have to kill a particular machine but you also can't be sure that every machine that you defeat will give you a lens. This means that getting the components may take some time. Remember that Snapmaws are vulnerable to fire and shock works great against Longlegs.
When you get both elements, the quest is updated and you must go to the Village to trade the items for metal shards.
Finishing the quest
After you speak with the merchants and give them the components, you get the money which the completes the quest.