I think this is a great topic because I have noticed this with specific MAC items and have become somewhat annoyed by their policy. Ultimately I believe this goes back to "you want what you cant have." By making certain MAC products Limited Edition (LE), they are in essence driving up the demand because they make limited quantities. From a marketing perspective this makes sense because it sets MAC apart from other business, ultimately giving them license to charge more for these items. What happens as a result of making a product LE is people over purchase specific items and re-sell on various forums such as EBay. Popular, harder to obtain items means sellers have an advantage in raising the price of an item. Take MACs Whisper of Gilt as an example. Most recently, this item, which retailed at $29/30 when released was being sold on EBay for over $100. That is absolutely insane!!! What makes this product worth that much? The market (Limited Edition) and the demand (not enough customers able to get this item).
One possible solution would be to stop making these items Limited Edition. This seems to unlikely given that MAC has a stake in the LE market. They really know how to capitalize on making customers anxious to buy the next big product which is released ona specific date and if you miss the boat then too bad! Compare this to another makeup brand that doesn't impose LE restrictions on their items. There is less of a demand at the onset of release and less likelihood that customers will pay exorbitant prices for these items because of their availability.
One possible solution to this dilemma (as previously suggested) would be to impose a restriction on the number of items one can purchase on the MAC website. Of course, by adding a restriction to the number of items one can purchase does not entirely solve the problem of customers buying popular items in large quantities and reselling at a higher rate. If you limit the number of items one can purchase online, that person will most likely seek other possible venues for purchasing these items (cosmetic counters, asking friends to purchase online).
MAC should recognize that creating LE products causes customers distress and imposes an unfair market share. Ultimately I believe they know what they are doing and don't seem to mind that customers re-sell popular items at 200% markup from the original value.
Just my two cents.