Inflation
From Tardis Wiki, the free Doctor Who reference
The Tremas Master travelled back to 1964 to purchase Who merchandise at pre-inflation prices to sell at a later date. This included Dalek slippers, Dalek soaps and Dalek loo rolls. According to the Sixth Doctor, it was this transgression of the First Law of Time resulted in the Master's banishment from Gallifrey. (NOTVALID: Doctor Who? 107)