This is something I've been thinking about too. The way I see it working is for the customer to be able to "re-order" a previous order - on the user page where you can see your previous orders (user/#/orders) there should be a button next to each order in the list which lets the customer populate his cart with every item from that previous order. Pushing the button *adds* those items to the cart (without affecting what is already in the cart) then takes the customer to the "cart" page. The customer could then check out right away or add/subtract items from the cart before checking out.