Alas, South Carolina was not in a position to sell Fort Sumter for back taxes. Otherwise the Civil War would have been the Civil Action (chuckle, snort).
The process that the U.S. used to acquire the land that Fort Sumter was built on was the same process used to acquire any land for the government. And the process was very simple. A state, South Carolina in this instance, through its members of Congress and its senators, argued that the National Interest required that a fort be built, such as one in the middle of Charleston Harbor. The necessary legislation would pass Congress and be signed by the President. Then the state legislature would pass a law granting title (if the state owned the property) or affirming title (if the land was privately owned) to the United States. It is important to note that then, and now, the government refused to accept property where there was any other restriction. The state gave up all rights it might have in the property. Otherwise Congress would refuse to appropriate the necessary funds to build the installation. The one general exception was a clause inserted to allow state officials to enter the Federal property to seize fugitives from justice or to serve civil process papers. And it is important to note that the title given to the United States was fee simple, with specific notice that the property was exempt from any state or local taxes. Except for the qualification that the property could be entered to seize fugitives, the property passed in perpetuity to the Federal government. Then, as now, states love it when the government pumps money into their cities so the necessary legislation was passed quickly.
So Fort Sumter was the property of the federal government completely and forever, unless the government decided to dispose of it.