In this case, website Sell a Style has tons of MAC-branded stuff on offer, including some "MAC" products that don't even exist, like Lady Gaga mascara ($6). They're certainly not the only site selling dubious branded makeup, and I wouldn't be at all surprised if a lot of these cut-rate cosmetics in similar packaging were showing up on eBay or even at some retailers' shops, presumably selling for about the same price as the real thing.
Although some of the products are clearly not real (the Gaga mascara and nail polishes in bottles that don't look anything like MAC's real ones, for example), the lipsticks and eye shadow singles are deceptively similar. And although they look just like your favorite MAC product, in reality there's no telling what's in them. That's why ponying up the cash for something you really want is better and safer than buying from some place that's not specifically cleared to sell products.
Just remember, unless you're buying from a licensed retailer, accurate-looking packaging does not mean you're getting the real thing; it means you don't know what you're getting.