Hi,
I've done a TON of research on Mac brushes and brush sets being sold on ebay. Well, basically, Let me tell you 80-90% of the brushes being sold on Ebay are fake. I went to the MAC store yesterday and bought some brushes and talked in depth with the sales girl. She told me that MAC DOES NOT GENERALLY MAKE BRUSH SETS MORE THAN 5 PIECES.   (except for the Pret-a-Porter set which had 8 brushes. see picture at end of guide)
The only SETS being made by Mac are sold around holiday time.
Here are the Heirloom sets. They each have a 5 piece basic brush set and eyes set.
They were limited edition sets. I think they retailed for $48. As you can see some of the handles are gold and some are black. There are 3 sets in woven clutches and sets in a cardboard cup. These brushes DO NOT have individual plastic sleeves with upc codes.





Now here is another brush set I believe was being sold this summer (July - August 08?) but they were exclusive to Nordstrom's. Again they are ONLY 5 brushes each set. They were available in blue, green, and red nylon bags.

 




Now, if you are looking to buy single brushes, here is an excellent picture guide I found for you to review. Pay particular attention to the close up photos of the black shimmer in the handle and the way the ferrule flares out.

http://risser.pixnet.net/blog/post/2080157


You can see the authentic brushes on the mac website. But don't totally rely on their pictures alone. Here is the foundation brush on the official website: 

But now you can see pictures of my foundation brush I bought yesterday at the MAC store at the mall:


They look different, right? But one is professional photography with good lighting too. My pics are pretty bad, but they are just to illustrate that the brushes may look different online, on other people's cameras, and in real life.

Also, I do believe all single brushes come in a plastic sleeve with a upc code printed on the plastic sleeve.
The handle is black with tiny sparkles in it. Despite many guides out there stating there are no sparkles. I JUST BOUGHT SOME BRUSHES FROM THE MAC STORE AND THEY ARE BLACK HANDLES WITH A TINY GLIMMER IN THE BLACK.
The handles have the word MAC written on them with the number of the brush. The MAC and number are located right next to the silver metal part of the brush (the part that holds the bristles in). I have bought all brushes from the MAC store and they are stamped Japan, China or USA. Here is a picture:



    Here is a picture of the only 8 piece set I found: Pret-a-porter. It retailed for $217 (limited edition, sold 2? years ago).



If you still believe Mac made 12,18,24 brush sets, let me show you a website. This website is for people from the U.S. or Europe to pay a company in China to manufacture fake Mac sets. Check it out for yourself. http://www.ec21.com/ec-market/mac_brushes.html

So, in closing, there are no 24 piece sets!!!!!! Even Mac sales people do not have any kind of 24 piece set or complete set. There's no such thing.
If you would like a good deal on brush sets, head out to CCO (Cosmetics Company Outlet). This is an outlet store owned by Estee Lauder. There are not a lot of locations, but if you have one near you, you are lucky. I bought a 5 piece brush set there for only $33.75. And I didn't have to worry if the brushes are fake!!!  Good luck. IF you MUST buy a set on ebay i think the best bet is to look for a limited edition 5 piece set (heirloom or colourforms, or whatever other are out there. (?). They all come with a  purse/clutch or cardboard cup. If you see a 5 piece set being sold without the clutch or holder, it may be just that the seller wanted to keep the clutch/purse. But ask lots of questions. The fakes are all coming out of China and i think it is too hard for them to counterfeit limited edition sets so they just make generic 24 piece sets hoping no one will know MAC never made a 24 piece set. If they tried to manufacture fake 4-5 brush sets then they would have to make fake purses too (the clutch or black with beads, etc) and that would be easy to tell if it's fake so they don't try it.

Hope this helps. Please vote for me if you found my guide helpful. Thanks!! =)