Wouldn't it just be a regular mailto: link?
Like this:
Wodri Clock - The screaming eagle. <a href="mailto:your@email.com">Buy it Now!</a>
You can also auto-populate the subject line for them.
Wodri Clock - The screaming eagle. <a href="mailto:your@email.com?subject=Wodri Clock - The screaming eagle">Buy it Now!</a>
The code would be added in the description of the image. The ones I use allow for html to be used. Otherwise you'll have to mess with the lightbox javascript.