Buchanan authorized a relief expedition of supplies, small arms, and 200 soldiers.....
Instead, it seemed prudent to send an unarmed civilian merchant ship, Star of the West, which might be perceived as less provocative to the Confederates.....
As she approached the harbor entrance on January 9, 1861, Star of the West was fired upon by a battery on Morris Island....
Major Anderson prepared his guns at Sumter when he heard the Confederate fire, but the secrecy of the operation had kept him unaware that a relief expedition was in progress and he chose not to start a general engagement.....
Beauregard made repeated demands that the Union force either surrender or withdraw and took steps to ensure that no supplies from the city were available to the defenders, whose food was running low....
The South sent delegations to Washington, D.C., and offered to pay for the Federal properties and enter into a peace treaty with the United States. Lincoln rejected any negotiations with the Confederate agents because he did not consider the Confederacy a legitimate nation and making any treaty with it would be tantamount to recognition of it as a sovereign government. However, Secretary of State William H. Seward, who wished to give up Sumter for political reasons—as a gesture of good will—engaged in unauthorized and indirect negotiations that failed.”
Various extracts from Wikipedia.
And noone else ever treated them as soverign either.
A bit of an exaggeration. It might be more accurate to say that the South sent a delegation to deliver their demands to be recognized. Only if Lincoln caved was there a vague offer to discuss "matters and subjects interesting to both nations." So if paying for the stuff they stole wasn't interesting to the Confederacy then it wasn't open for discussion.