Should the purchase of mass, spree or serial killer paraphernalia be made a crime? why or why not?

It shouldn't. The United States are a free country when it comes to economy which means that supply and demand decide the market. If people want to buy something there is no reason for them to be stopped from doing it unless the items in question are used in an investigation. If the items are out of an investigation and are just common items now with history behind them, then the government should not ban people from procuring the items.