The three biggest improvements eBay could now (realistically 😄 ) make to the site would be to get rid of the "Buy Now" button so that there is only 'Add to cart', plus make it so that clicking 'Add to cart' doesn't load the cart, but keeps you on the same page so that you can continue to easily navigate a seller's store, plus not only should you be able to see that the number of items in cart via the icon on the top right, it would be neat if you could mouse-over it for a quick summary of those items.

IMHO, that would cut down a lot of the separate payment issues. The postage total (where applicable) could probably stand to be updated and more clearly indicate that it's a preliminary figure if auto-discounts aren't in place, with a prominent 'request invoice' button.