I've been reading a lot of the guides here on how to spot fake MAC cosmetics and almost all of them are quite good.

So, I'm not really going to go over what they've mentioned already but just adding some more information.
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In another post, this particular paragraph was mentioned:

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Fifth: MAC Cosmetics do not have defective products and then sell it at a lesser cost. They do not participate in Gift With Purchases or pre-packaged samples. The closest thing to this is a Estee Lauder Cosmetics Company Outlet but they only sell some products and not at a super deep discount.

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I'd like to clarify though that the actual MAC Cosmetics Ltd. don't sell their defective products for a lesser cost but instead the authorized department stores who clears their surplus inventory and sell it to liquidators.  Some maybe new, shelf-pulled, or returns.  Also, in Toronto, there is a Clinique/Estee Lauder/MAC Warehouse Sale that happens 2-3 times a year.  This is by invitation only to family and friends of employees but some sell their tickets instead. Prices are low and they give a few freebies.

Also, in the beauty industry, its "who you know" that can help you get the products you want. 

Mostly for MAC, the lowest pricing you can get on authentic product is only 30-50 % discount, and that depends on how direct you are to the true source.

Anyways, hope this helps....Thanks