I want to take a second and underline the point that u/Kindapuffy made about not everything being a good deal. Amazon is well known for fake makeup, but they do have a good number of legitimate brands. If you check the item page and it says ships from Amazon, it's probably okay. Items shipping from 3rd party sellers are highly questionable, especially for mid-range to high-end brands.
We know there are some brands that Amazon sells that are legit, like Lime Crime, Jouer, LORAC, Stila, etc. but there are also fakes for these brands available from 3rd parties on Amazon, as well as resellers who might not be selling fakes but aren't selling you merchandise direct from the company or from Amazon.
To make sure what you're buying is legit, check the following:
Who is the seller? Who ships the item? Is it the company (LORAC, Lime Crime) or Amazon, or is it a 3rd party?
Is the non-sale price listed the same as the original price if I were to buy this item at Ulta/Sephora or direct from the company web site? If it's different, you might be looking at a fake or reseller.
What are the reviews like? Does anyone mention fakes or resellers?
How did I find this product? If you found it through a brand link on Amazon's Luxury Beauty page, it's likely fine. If you found it through searching for the brand or product, it could be fake, so be careful.
Examples: