Reasons Why Buying Essential Oils "Just Anywhere" Can Be Problematic
Reason #4) Oxidation
Oxidation is the process of the breaking-down of molecular structure. All essential oils oxidize. Some oxidize faster than others (see Reason #2) Shelf Life). There are three things that cause e.o.s to oxidize; heat, air, and light. Therefore, the best case scenario for storing them is in a cool, dark place in with tightly-sealed, dark glass containers.
Essential oils sitting on a store shelf are exposed to heat and light which are only going to degrade their properties faster than in ideal conditions. If those pretty little bottles sitting exposed on a shelf have no documentation of when or where they were produced, you should understand that the properties have been waning away the whole time they have been there and you will have no idea what is in it, short of doing your own testing.