Didn't work out so well for the Confederate states in 1861. Their currency was worthless in a very short space of time, mainly due to the Union Naval blockade restricting the export of cotton.

If there was a civil war in the US again, it would probably go badly for the side that couldn't get access to international markets. But I don't believe a second civil war would be one based on the right to cecession, it would most likely be a fight for control of the Government and be a protracted asymmetric guerilla campaign funded by drug money, and the proceeds of terrorism.

It would be nasty and brutal.