Mandelbaum v. The People was a case heard by the United States Supreme Court in 1869. The case involved a dispute between the plaintiff, Mandelbaum, and the defendant, The People, over the ownership of a piece of property. Mandelbaum claimed that he had purchased the property from the defendant, while The People argued that the property had been sold to another party. The Supreme Court ultimately sided with Mandelbaum, ruling that he had acquired the property through a valid purchase. The Court held that the defendant had failed to prove that the property had been sold to another party, and that Mandelbaum had acquired the property through a valid purchase. The Court also held that the defendant had failed to prove that Mandelbaum had acted in bad faith in acquiring the property. The Court's ruling in Mandelbaum v. The People established that a valid purchase of property is binding, even if the seller had previously sold the property to another party. The ruling also established that a purchaser must act in good faith when acquiring property, and that the seller must prove that the purchaser acted in bad faith in order to invalidate the purchase.

In the case of Mandelbaum v. The People, the Supreme Court was asked to decide whether a state could constitutionally impose a tax on goods imported from another state. Justice Field delivered the dissenting opinion in which he argued that such taxation violated both the letter and spirit of Article I, Section 10 of the Constitution which states that “No State shall…lay any Imposts or Duties on Imports or Exports” without consent from Congress. He further noted that this clause had been included in order to protect interstate commerce by preventing individual states from enacting laws designed to favor their own citizens over those living elsewhere. In his view, allowing one state to impose taxes on goods coming into its borders would create an unequal playing field for businesses operating within different states and thus undermine free trade between them. As such, he concluded that it was unconstitutional for New York to levy taxes against goods imported from other states without congressional approval as doing so would be contrary to what was intended when this provision was added into our founding document.