I haven't looked into Zencart yet, but i was wondering if I use Zencart in the near future how easy is it to intergrate it with Zenphoto?

Just a thought! has anyone done this?

You could look at the zenPaypal and GoogleCheckout plugins. I know nothing of Zencart but I presume it could be integrated in a similar fashion.

Since Zencart is a independed shop solution I doubt that a plugin will do it. I only know it by name, but I guess it will best work as separate independend installs.

I myself recently was trying to decide whether I should use zencart or zenphoto. Zencart is a whole shop cms, and since I only want to sell my photos then zencart is a bit overkill. I love zenphoto especially using my theme, and achieving this same behaviour and look in zencart would be too much work. It was much easier to just add paypal integration into my zenphoto. I would, however, like paypal integrated a bit better, so if I would want to I could specify prices for each photo manually and turn off paypal for some albums/photos.

You can set specific prices for each image by using the custom data field in the admin:
http://www.zenphoto.org/documentation-official/zenphoto/_plugins---zenPaypal.php.html

You could easily tie the display of the paypal button to the custom data field too with a simple if clause. For example if it contains "no paypal" don't show the button. Just an idea.