M.A.C. (Makeup Artistry Cosmetics) is a Canadian company.  For several years, they've been making cosmetic pigment powders.  Samples of 1/4 to 1/2 tsp. of these powders have become increasingly popular on Ebay.

Pigments come in a huge array of colours, shades and finishes.  They are commonly a multi-use product.  They can be used as eye-shadow, blush, lip colour when mixed with gloss, nail colour when mixed with polish, hair colour when mixed with clear gel and even body colour.  Some of the colours have been determined to be unsafe for the eye area, and a search of the web can point you to a specktra-l site which lists them all.

When purchasing pigments from M.A.C., they come in small containers that hold approximately 3 teaspoons (depending on the weight of the individual pigment) and retail for $19.50 USD. 

One of the reasons pigment samples are so popular on Ebay is that a full-sized pot of pigment would last forever, due to its heavy pigmentation (color payoff).  Its quite common for someone to purchase a full-sized pigment and remove 1/4 teaspoon to 1/2 teaspoon samples for personal use and then sell off the rest. In addition, a sample is good for 20-30 uses many times, and allows the user to try out many different colours and finishes before deciding to make a larger purchase.

On occasion, M.A.C. pigments are limited edition colours (LE) and therefore they will usually sell on Ebay for much higher amounts than "permanent" items in the company's inventory.

When used on the eyes, pigments can and DO crease and fade, despite the claims of the company.  A good eyeshadow base is highly recommended when using pigments, and there are some very good brands of drugstore eyeshadow base that sell for very little.

On Ebay, pigment samples are commonly sold in 5 gram plastic pots. To ship one pot is generally $1.00 USD or less in the continental US, and in order to reduce shipping costs, buyers should try to purchase more than one sample at a time.

There are other companies that make loose pigment powders, the drugstore brand Milani and a company called Bare Escentuals are but two examples.  M.A.C. is the gold standard though because of consistently gorgeous colours and that outstanding colour payoff (pigmentation).

Pigments are versatile, fun and widely available on Ebay.
Enjoy!

Update: Evidently, Ebay US has determined to no longer allow sales of samples of M.A.C. Pigments. I think this is a crap move on their part, but do you think they listen to ME?  Anyway, you can still buy them on Ebay UK, but be prepared to pay a much higher shipping rate.