Using the new selectbox, you can select multiple photos from multiple albums and perform batch operations on them.
As can be seen in the screenshot below, the photos are selected and deselected by clicking a checkbox in the upper left corner of a photo within an album. You can also see that there are four buttons on the selectbox : "Owner's Tools", "Buy", "Clear", and the minimize button.
The "Owner's Tools" button gives you - as the owner - access to the common batch operations you may want to execute on the selected photographs : access mode change, geo-location update, description & keywords update, featuring the photos, and reorganizing them within the album structure.
The "Buy" button works the same way the classical "Buy" button but you need not select the photos, since they are already chosen and the best thing is that they can be chosen from multiple albums instead of only one.
The "Clear" button simply clears your selection and hides the selectbox, since it has no use without any selected photos. The last button minimizes the selectbox so that only the number of selected photos and a link to restore the selectbox are visible. The minimize button is especially useful when navigating between albums, since the selectbox is visible wherever you go.
You can have both, yours and others' photos, selected at a time and still use the "Owner's Tools" to manage your photos and the "Buy" button to add products of all the selected photos to the cart.