What was one reason south carolina was unable to trade its cotton for war supplies?

Slavery--since the American South still used slavery to produced cotton many countries did not want to use Southern cotton on moral grounds. 

As the Civil War became more and more about slavery, the world did not want to support an immoral act. Great Britain had already outlawed slavery and though they wanted to trade they didn't want to look like they supported slavery. Also cotton was not as valuable as it once was because of over production and more availability in the world. Manufactured goods like guns were worth more money.