When buying, it may be easier to "select" the thumbnails of the photo's you want to buy, rather than going through the individual photo's when they are large.
To select photo's to buy when looking at the larger photo's, "hover" over the buy button that is on the upper right hand side and select "save for purchasing later".
Once your selection is made, you can view your selections by going to "selections for purchase" that appears at the top of the window on the left hand side. To buy, select "new product", you can then "check out" from there.
Clear as mud? Email or call if you have any probs at all.