Pure oils are very important. Reputable suppliers will label an oil "genuine", "genuine and authentic", or "G&A". These labels indicate that the oil was extracted from a specific plant species, and was extracted in the method most beneficial to that plant, and that the oil has not been diluted or adulterated in any way. Labels like "100% Pure" and "Natural" are applied to oils that are extended with a vegitable oil (a few drops of rose mixed with an ounce or so of olive oil is still natural). Some of these oils are extended with synthetic fragrances - rose oil, to stick with my example, does not smell exactly like a rose flower, so some manufacturers will blend in a little something else to make it smell more "rose-like". Also, these oils may be mass produced from a lot of different kinds of roses, rather than just rosa damascena, the preferred rose for essential oil. Or worse, they could have harmful chemicals left over from the chemical distillation, or used as extenders and fixatives of the fragrance, which makes internal consumption of the oils even more risky.
So, look for "G&A" after the name of the oil, look for the biological name of the plant - and in many cases, point of origin. Look for widely varying prices. Check a drop of the oil on your skin, if you can, to see if it feels oily. And buy from someone who's reputation you trust.