If a seller has run out of stock on a product, you will have the option to be placed on their waitlist. All you need to do is provide the quantity you will want and your email. You will be emailed when the product is back in stock so you can purchase.
This is a way for you to bookmark products you are interested in until a later time. All you need to do is look for the wishlist button on the right side of the page and click it.
After you add it, the heart will fill in.
To view the items on your waitlist, click on your profile area
And click on "My Wishlist" which will direct you to the page. There is no limit to how many products you can add to your wishlist!