Add to cart is a preliminary purchase function. A buyer can add several things from several different sellers to their cart, and leave them there indefinitely for when they want to purchase, or go through checkout and buy/pay for them all at the same time (if paying by PayPal for them all, anyway, and as long as the items are all listed in the same currency).
If a seller has postage discount rules set up, they will automatically apply and display the total in the cart if they have multiple items from that seller, but if not and the buyer will need to wait for a combined invoice, it's better for them to buy separately (without adding to the cart, because once an item is in the cart you can only proceed directly to checkout and pay).
