First I'm no expert. Guide is my response to someone who asked me

 Where do you get your essential oils and how do you evaluate or qualify its purity?

Some Essential Oil Buying Tips:

1) Glass bottles (amber or cobalt) are best for storage. Thus if SOLD in these glass bottles, indicates higher quality seller. Aluminum bottles are ok if inside is lined.

2) Many Ebay sellers do not know the difference between fragrance oils FO and essential oils EO. Lots of fragrance oils are mislabled as EO. Of course, occasionally you can get a deal on EO from someone that has no clue. EO is much more expensive.

3) Buy in large size for best price. Buy small kit to try out.

4) Look for scientific name, country of origin and method of manufacture. I don't know much about oil quality except lavender. Lavender has a couple of different formulas and locations. Make sure you are comparing formulas /locations when comparing price. They have the same formula under a couple of different names. There are also multiple varieties of Bay, Cedarwood, Chamomile, and Eucalyptus. Each has different therapeutic properties. The country of origin for oils is also important because the climate and soil conditions can affect the resulting properties of the oil. Is that rose oil steam distilled or is it an absolute?

5) You could look for a good supplier that has periodic specials.

6) Look for differences in price. Neroli, Jasmine and Rose, should cost a lot more than geranium and Ylang Ylang. A good quality patchouli usually costs more than eucalyptus. The basic citrus oils such as grapefruit, lemon and sweet orange oils are some of the least expensive oils.

7) Avoid the words perfume oil or fragrance oil or aromatherapy oil, as these are very likely synthetic and diluted. Even pure essential oil wording can be misleading. look for manufacturing method, etc,

I first bought a small kit from Gemlite. They are not the high end seller, they do have great prices. So I first purchased inexpensive...to try as many essential oils..then for re buy I researched the formulas. Honestly I am not sure I could judge quality.

Manufacturing methods:

Today, most common essential oils, such as lavender, peppermint, and eucalyptus, are distilled.

Most citrus peel oils are usually expressed mechanically or cold pressed. Citrus oils  are easily manufactured and are inexpensive..however...they do NOT keep well, they go rancid. So use citrus in cleaners and not personal products in my opinion. Some Citrus oils can make you sensitive to sun, yet another reason not use use in personal products.

Another method, used for flowers is solvent extraction.

Some people worry about pesticide residues on essential oils and thus want organically grown.

Vendor Selection:

Be careful when buying essential oils from companies that primarily sell to the food & beverage or perfumery industries. Some vendors that primarily sell to these industries may have different goals in the purchase and sale of their essential oils than the goals of vendors that sell therapeutic-grade oils specifically for aromatherapy use. The restaurant and perfumery industries desire essential oils that have a standardized (consistent) aroma or flavor. The oils sold by these sources may be redistilled to remove or add specific constituents (natural chemicals found in the oils). These re-distillations or adulterations may harm the therapeutic use of the oils. If desiring to buy from such a vendor, inquire first to ask about their methods.

Some oils are blended to avoid differences. Some lavenders are blended.