“Why would sellers of southern cotton sell it without being paid for it?”
They wouldn’t. Southerners repatriated their wealth from overseas, mostly in the form of imports.
It was the confiscatory rate of taxation on imports that caused a problem.
Bingo.
1) The Exporters WERE the Importers.
2) As Imports were reduced by the tariff, that reduced the income of Britain/France. They in turn could therefore afford LESS American ie Southern exports (imports to them) since their income was reduced. So as the Southerner, you sold LESS Cotton/Tobacco AND you got hit again because the manufactured goods you filled the holds with those ships making the return journey across the Atlantic were now subject to much higher tax. So you bought fewer of those goods and kept more cash to hand over at the port of entry.
Same amount of Cotton/Tobacco but now less demand for it, prices went down.
Southerners lost sales, paid higher taxes, got lower prices for the goods they exported AND THEN paid higher prices for manufactured goods. So for them it was Lose-Lose-Lose-Lose. Damn right they were angry about it. Especially when they knew it was all artificial. It was all for others' benefit. The cherries on top of this chit sundae were the they only got a very small share of the tariff money paid in the form of pork the federal government doled out and finally, they no longer had enough votes to stop this legalized plunder...and they knew that.
That's why they left.
They didn't leave to "protect" something which was not threatened and which only a very few of them participated in to begin with. It would be like the government saying you are no longer allowed to own a Ferrari. I'd be pissed because who the hell are they to tell me what I can and cannot buy with my money but realistically, I'm never going to be able to afford a Ferrari anyway so I'm certainly not going to be willing to fight over it. How about if the government started taxing me out of house and home? Now I'm ready to get my guns.