How Buy Makeup Online: Staying Safe and Healthy

I have bought and sold makeup online, and have a background in healthcare and make up artistry.
I have also had some GOOD and BAD experiences with online sellers. Here are some basic health and safety guidelines for  the various types of beauty and skin care items founds for sale online...

TESTERS:  First understand that "TESTERS" are from stores or direct sales companies and may be fine and safe to purchase. Understand they are not the product packaged in a manner to protect the consumer (YOU!) This can lead to problems as some "TESTER" items may have been contaminated either by accident or by actual swatching / testing on skin, which is as we know, is covered with germs, bacteria and so forth.
I have had good experiences and some bad with "TESTERS".  For example, after receiving what I thought was a "new lipstick", I found that the packaging did not match what I saw in the picture in listing, or on the website of the manufacturer. Instead of a being sealed, and having a full cap, It sort of had a half cap, and had had no package or any seal, that was my OMG moment, that I had paid for a "TESTER"  not a REAL, NEW lipstick. Testers are available to those who work in the makeup industry. The seller may have good intentions but the truth is, there is no guarantee that what you receive will be safe, unused (it is easy to cut off use marks) or been touched by many dirty fingers.  That is unless the seller can vouch for the TESTER with actual pictures, and the story behind it, and perhaps offer to sell only sealed, unused TESTERS or some guarantee of the sanitization process used to protect the buyer.
Testers may carry germs (of course you can't see them and it all looks good) until there is a breakout, pink eye (that is just one of the eye infections that come from contaminated items) or a rash that is bacterial related.
If something about the product picture looks "off" or "just not right" send the Seller some specific questions so you know if you are buying a "TESTER" version of a product.  "Testers" may be a  great buy, unless has been contaminated.  Than it is or can be a host / medium for bacteria (germ) growth.

Used or Gently Used Makeup or Skin Care Products:  If you were at Starbucks and the person next to you had a delish (fill in the blank) an you have always wanted to try it, REALLY wanted to just see what it was like. They see you admiring the (fill in the blank) and then sneeze into it, maybe twice, and also cough all over it (no they did not cover their mouth) and they are offering it to you to try, also while putting their fingers into the contents of the container to hand it to you... at a really good price, free or in the case of "used makeup" or "skin care products" at  really discounted prices...so tempting....
99% of us would be polite and say thank you but, NO THANK YOU.
Buying used makeup, gently used, used, half or partially full or otherwise is taking serious risk.
That risk is applying or ingesting, via the used makeup or skin care product: BACTERIA, VIRUS, FUNGAL, PARASITES (that would be teeny tiny worms) and other dangerous particulate matter. These can make you very ill,  cause an infection or at the minimum cause a dermatitis (rash) that will require antibiotics and a doctor visit.
Buying previously USED makeup and or skin care products is a roll of the dice as you can see from the info above.
The one exception I can think of,  to the above warnings,  is If it the Seller is an obsessively/compulsively hygienic consious makeup artist who knows how to, and has the industry solutions, to spray on makeup (hospital grade stuff) to "clean"  or "sanitize" makeup, before using it on the next client (excellent hygiene is a sign of a great makeup artist, usually they do not even touch the makeup, using tools to put it on a palette for each client, and then a disposable applicator or different brushes for each client so there is NO contamination.  
The problem is most used makeup is not sold by obsessively hygienic makeup artists who have PROPERLY sanitized everything.
You are most likely looking at used makeup from a busy seller, who does not sanitize makeup, and may be selling makeup that could be teaming with contagions.
With MRSA, VME and other antibiotic resistant strains of bacteria, and viruses that love to grow in makeup, I would pass on and say NO to most "used make up" or "skin care" products.
Especially as many people keep or take their makeup into the bathroom / toilet area where e.coli  (yes it is gross to think about, however an issue, especially if your seller is not a big hand washer...what can I say.)
You have to make the decision, is it worth the risk of getting sick, and the doc bills that will follow???
At least buying from listings that ensure make up and skin care is new / sealed, you know you have the quality controls of that company, which you can research, protecting you.

Sample Sizes:  If it is sealed, and new in packaging that has not been tampered with, you can pretty much trust you will not be risking an contamination issues.
The one thing to think about is the PRICE of the sample compared to the actual price per ounce (or full size) and is that worth it.  That is where a little research may help a great deal.  Makeup Stores and many Dept. Store counters are starting to "get it" --- we want to try before we buy.  Why pay for something someone else got for free, when you can also.  Finally, if all else fails, and for some reason you cannot get a free sample, look for a reputable seller when dealing with these teeny tiny amounts of product.

Perfume and Fragrance:  We all know that we should NOT store our fragrances in direct light, on top of a counter, or anywhere light can get to it and break down the smell we love so much.  Does your seller know that? Where was what you are looking at stored? It is a small thing, and the bottle may look perfect, but if it has been in direct sunlight for months on end, you will probably notice that the smell is not what you expected. So ask the seller!

Used or Vintage Fragrance:  Again, where was it stored? I would be a little less concerned about contamination due to the alcohol content of many fragrances, which will generally keep it sanitary to some extent. However, if there is no "spray" mechanism, that increases the likelihood that the seller or others have touched (contaminated) the opening of the bottle.  Is it going to harbor germs and bacteria like makeup can? Perhaps especially if it is a scented oil based product, that then forms a medium (place for germs to grow) on the neck of the bottle. Ask questions of the seller, find out as much as you can, and then make the decision to buy or not.