I get this quite often and this is the bandaid method which has worked for me but onlt 50% of the time to restore it back to normal; (until next login)
Open the Black Market
Select the Buying tab
Find the cheapest item (or something you don't mind buying)
Press the Purchase button, NOT the cart icon
then in the checkout pane press the Checkout ($XX.XX) button
Now go to the selling or buying tab and select the item and adjust the quantity and checkout like normal, it should now calculate and show the total cart and allow you to checkout out.
My experience with the bug is anything that is in the shopping cart pane which includes using the shopping cart icon but not the Purchase button in the Buying Tab and all selling buttons in the Selling Tab. When trying to process the shopping cart it is not calculating the cart and moving to the checkout pane (sometimes spam clicking will make it work).
The only button that does work is the Purchase button because it goes straight to the Checkout pane.