This is a category where what you can see decides what you should pay, because nothing about a cosmetic can be verified from a description.
Ask to see
The seal, intact, on camera. The batch code, close enough to read. The expiry or best-before, where it is printed. The open-jar symbol — the small jar with a number and an M, which tells you how long it is meant to last once opened.
A seller who shows all four without being asked has done this before.
Then ask two questions
"Where did you buy it?" The answer is the provenance, and it is the only provenance a cosmetic has.
"Has it been opened?" And if yes: when, and how much is left.
On authenticity
Nobody authenticates a cosmetic over a video call. Counterfeiters target this category hard, so weigh the price against how much the seller has actually shown you — and treat a confident "it's genuine" with nothing behind it as worth less than a careful "here is the code and here is where I bought it".
Two things to refuse outright
Decanted or repackaged products. If it is not in its own container, do not buy it. Anything sold with a claim that it treats or cures something. That claim is not one the seller can support, and medicines and supplements need a licence the seller must hold. Prohibited items.
What protection covers
A product that arrives unsealed when it was described as sealed, past its date, visibly different from the one shown, or not as described in any other way: seven days from delivery.
Questions people ask
Is a product past its best-before unusable?
Not necessarily, but it is worth less and it must be declared. Decide with the date in front of you.
What does 12M in the jar symbol mean?
Twelve months of intended use after opening. It starts when the product is first opened, not when you receive it.
Should I buy used makeup?
Some people do and many will not. If you do, buy only where the seller says plainly when it was opened and how much is left.