The property didn't belong to them, or at least to them alone. They took it. They didn't offer to pay for it before they took it. Sure sounds like stealing to me.
Apparently they did offer to pay for it, although it was a Godfather-like "either your signature goes on the contract or your brains do" sorta offer. From the letter that Buchanan's Sec'y of War sent to the SC AG:
The proposal, then, now presented to the President is simply an offer on the part of South Carolina to buy Fort Sumter and contents as property of the United States, sustained by a declaration in effect that if she is not permitted to make the purchase she will seize the fort by force of arms. As the initiation of a negotiation for the transfer of property between friendly governments this proposal impresses the President as having assumed a most unusual form.What else is interesting here is South Carolina's admission that Sumter is, in fact, United States property.