The North was perfectly happy with the arrangement where THEY bought the cotton and resold it to Europe; manfactured the majoritiy of the products sold to the South; bought and resold products from Europe and reselling them to the South; AND performing the costly transportation for all of these transactions; PLUS the bulk of the revenue was collected in the North. It was a great deal.
Add a trade emport tariff and you got the perfect economic monopoly. The South would be forced to pay higher prices to the North for shipped goods (almost all their bought goods were shipped to them); all the revenues would be collected in the North being a foriegn emport tax; the South would have to buy more Northern goods instead of what had been cheaper European goods; and the North would still be the primary buyer of Southern Cotton, selling it at an inflated price to Europe after tacking on market and shipping cost.
The Northern banks held most of the Southern loans.
The North bought, shipped and sold the cotton.
The North manufactured most of the products, grew most of the food and sold and shipped them both to the South and Europe.
The North collected and revenues for the majority of the transactions.
The North controlled the prices of their goods, protected by a Federal Tariff, bolstered by Federal subsidies and an international shipping monopoly.
The South was 100% being economically exploited by the North and the North could not afford to let any part of their cash-cow go by secession.