It will help you to know what items are used in which quantities. If you know it takes 3 logs to synth 1 lumber, you know 3 is the min, and multiples of 3 raising up will help. Some Ingots take 4 pieces of ore to make 1 ingot, which means sets of 4 will help more. Leathers tend to be qty's of 1's. There are some patterns but, as a whole everything is so different in total pieces needed per synth, it will take a bit of looking or having the crafting logs open in order to get the optimal sales quantity. I usually always buy in proper sets as a crafter just so I don't have to store or resale extras. My inventory space is more valuable to me hence I will pay more for the proper amount.