There's a responsibility here on both the customer and the store owner as well. Customers should be willing to let their store know they're going to Gen Con and may/will be buying items there. It's a courtesy a regular should give their local shop. And, store owners should know their customer base. If those 15 copies of the book are mainly going to the 'regulars' who play Malifaux and a few of those regulars are going to the con, odds are most/all of the regulars will have bought the book there. Ask and adjust initial order numbers accordingly.
When players are intimidated about communicating with their LGS, and the LGS doesn't bother to poll its regulars then there is going to be some stock collecting dust post-Gen Con. That'll happen whether or not Gen Con vendors adjust their plans or not.