When I'm feeling economical, I peruse the websites of my favorite beauty brands, hoping for steals on discontinued products. I've had good luck with Stila, DuWop and Benefit, but almost every beauty brand has a section on their site for "vintage" items. As you can imagine, the stock is in constant flux. But if you check often, the odds of finding a great deal are in your favor.
Currently available from the DuWop vintage shop is ICE, a pale pink pressed powder. I bought this shimmery confection full-price, when it was introduced as a limited-edition must-have two winters ago. Although I love the product (despite being a very cool shade, it works as a great highlighter on both warm and cool skin tones), seeing it available now for 20% off with free shipping is a great reminder of just how much of the beauty industry is about creating hype.
Be discerning when it comes to limited-edition items, unless it's a new Chanel nail polish. In which case, stockpile those bottles to your heart's content.