Looking on eBay I can't believe how many fake goods there are - Nars, Bobbi Brown, MAC seem to be the big targets. I can spot them a mile off because I am a beauty junkie and I am familiar with packaging and shades but there are plenty of people buying the fake stuff too. I'd say be wary when buying branded goods, esp things like Brush sets that are 80% off the retail price! It doesn't make sense. After all, its not just the fact its fake, its the fact that you don't know what chemicals are in there either.
On eBay you can always get a refund if you get a fake item, and make the seller refund the postage too, if you pay with Paypal!