I recently purchased some MAC brushes on Ebay that turned out to be FAKES.  This guide will help you spot counterfeit MAC makeup brushes. 

MAJOR DIFFERENCES BETWEEN AUTHENTIC AND FAKE MAC BRUSHES

1.  PACKAGING
    Authentic MAC brushes come in a clear plastic sleeves with a bar code printed on the sleeve.  Many fake brushes I have seen come in clear plastic sleeves without the bar code.  In addition, many fake face brushes come with additional plastic protectors for the hairs.  Real MAC face brushes, including the 168 and 190, do not have this additional plastic protector.  They come in the simple clear plastic sleeve with barcode. 

2.  HANDLE
    For those who received the brush already, examine the handle of the brush.  Many fake MAC brushes come with black wooden handles that contain tiny shinny particles.  Authentic MAC brushes have matte black handles.  The fake 239 brush I bought had the shinny black handles whereas the authentic brushes purchased from Nordstrom had matte black handles. 

3.  LETTERING
    Authentic MAC brushes are labeled "MAC ###".  ### being the number of the brush.  On Authentic brushes, the lettering is printed on the handle and easily rubes off with your nail.  On most FAKE brushes, the lettering is engraved into the handle and very hard to rub off.  Try it. 

4.  BRUSH SETS
    Keep in mind that MAC has never offered a set of full sized makeup brushes except until November 2006 called the Pret-a-Porter brush set.  This set includes 8 full size brushes (187, 190, 129, 213, 224, 266, etc).  The set comes with a black satin brush roll.  So if you see a full size MAC brush set on Ebay but it doesn't have the 187 skunk brush, it's FAKE. 
    Therefore, any auctions for full size MAC brush sets must be examened carefully.  The typical auctions for 8 MAC brushes with black brush case are FAKES. 
    MAC does make many travel size brush sets with brush numbers ending in SE.  *UPDATED* - The last photo below is of a fake travel size MAC brush set I found on Ebay recently (Jan. 2008).    Authentic travel brush sets usually come in sets of 5 brushes with a cute brush case, the brush handles are usually some unique color, not black.   The fake set below, however comes with plain black handles and a generic looking brush roll. 

5.  THE SELLER *NEWLY ADDED*
    Never buy MAC products from a seller you don't trust.  Sellers to watch out for are a) Newly registered sellers with no or little feedback, b) sellers in other countries such as in Asia, Australia and Canada, c) sellers selling a lot of other MAC products. 
    Please do yourself a favor and click on the "View Sellers's Other Items" link on the right hand over below the sellers info.  If the seller is selling other MAC or designer makeup products that week, ask yourself how this person gets their hands on cheap makeup products.  99.99% of the time everything that person is selling are FAKES. 
    FEEDBACK ISN'T ALWAYS RELIABLE
    Plenty of people buy fake MAC brushes on Ebay without realizing it.  They leave positive feedback for the seller and in turn convince more victims to buy.  There are plenty of sellers with 99% or higher ratings who are selling fake MAC brushes.  Bottom line - don't trust feedback.  Do your research instead or buy from a store.  Don't try to save money but end up paying for a fake brush set. 

Stock Photo of the Pret-A-Porter Brush Set (Authentic). 



EXAMPLES OF FAKE MAC BRUSHES AND SETS



(in the full size image you can see how the handles are a bit glittery and not matte)

(note the plastic cover that's not found on authentic brushes)

Full sized images here:
http://i13.tinypic.com/42midcx.jpg
http://i17.tinypic.com/3z8dgna.jpg
http://img.inkfrog.com/pix/number10/IMG_0671.JPG

*UPDATE* More Photos of FAKE brush sets