What probability of the drugs being counterfeit would you be willing to accept? Also, the profit maximizing strategy of the seller might be to initially sell 100% genuine drugs and then slowly lower this percentage.
It might. Another profit maximizing strategy might be to gain a large portion of the market by making her business open, reliable, and with a quality product that grows through word-of-mouth advertising.
Based of what I've heard from and about the seller (that she responds to emails, offers refunds, sent gwern free samples, and keeps a running blog on the site), I'm going say that rather than a manipulative game theorist, she sounds like a genuine businesswoman who's found an open niche in the market.
Due to agreements with the patent holder, Cephalon, that were made in 2005-2006 several generics manufacturers are now allowed to sell generic Modafinil. I have found confirmation that the manufacturer Teva has begun selling generic Modafinil, though I haven't seen news about other manufacturers. However, the article also, says that Cephalon is a subsidiary of Teva, so perhaps this won't have an effect? The Modafinil patent is set to expire in April 2015.