It takes you straight to a page where you can add that item to your cart. You can also use
this direct link if you'd rather visit the product page and add it from there.
Again though, these links are not going to show results if you come back later and Air Foto no longer has those items in stock.
The item was out of stock. If it's not showing in the seller's store, you will not be able to add it to your cart or buy it. Try refreshing the page next time and you will see that it really is not available.