This "purchase" feature should not be mistaken as a "shopping-cart" or "e-commerce" system. It simply allows external payment links per item from your gallery.
The purchase links are created from the admin, and you can choose to display the "Buy now!" link a long with the title/description, or in a separate location. You can also use the "purchase" imagebutton that appears on image rollover and displays a shopping icon. Clicking any of these buttons/links would take visitor to a separate purchase page as specified in the link.
How to set it up?
First of all, you need to go to admin -> config -> settings.attributes.image and add the "purchase" item. Save your document, and proceed to the management section.
Now when you select any image in admin, you will see a new field "purchase" below title and description fields on the right hand side. Here you can fill in an external payment link for the item. It might look something like this(fictional):
Code
https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=987654
Finally, you need to enable the purchase link to display somewhere in your gallery. You can do one or both the options below:
#1 Go to config -> image.imagebuttons.buttons, add the "purchase" item. This will create a new "purchase" button when rolling over the image, that can be clicked to purchase the item.
#2 Go to config -> image.text.text1.items, add the "purchase" item. This will append a "Buy now!" text link to your description. If you want to display this link in a separate location, you can enable it in either the text2 or text3 textmodule instead.
In both options above, default text displayed is "Buy now!", but can be changed through the language file editor.
Try your gallery and see if it works!