The PA Department of Revenue has issued a bulletin on how they are defining "nexus" in terms of the collection of sales tax. They're looking at you here, Amazon. Forbes has a story on the potential repercussions of this bulletin, which include "taking enforcement action and could pursue merchants for back taxes." Since the law the bulletin cites as relavent here goes back to 1971, the amount Amazon would be responsible for could be quite significant. Amazon has four warehouses here in the Keystone State.