I've done some research, and WooCommerce for WP appears to be a great plugin - especially since it has a plugin for Skrill/Moneybookers, which I use as my credit card processor already.
What I envision is to have a button under each photo, one for "limited edition prints" taking to WooCommerce (WC? :P ) cart, one for "open edition prints" using FAA cart. Of course the customer wouldn't be able to mix open and limited edition prints, but I guess that would require way more coding skills than I have, or quite an investment in a web designer.
As for themes, Vellum looks like a great responsive theme, and has native WooCommerce support:
http://themeforest.net/item/vellum-responsive-wordpress-theme/6914246
WooCommerce have themselves several themes which should guarantee good integration, and Maximize seems to be nice.
http://www.woothemes.com/products/maximize/
@David B. Yes, I read about the requirement for .ORG for WP when doing a search before posting yesterday, but it's good to remind about that. I prefer self-hosting, anyway. I like your minimalist design, as that's a similar direction I'm leaning to: a photo, and two links below it taking to the carts.
@Dave Your site is still gives 403 for me. Since you are blocking IP addresses, perhaps you have blocked entire ranges of them - that might block an entire country if you're not careful! I'm no expert on this, but I'd imagine IP blocking will not work anyway given how easy it is to use a VPN.
@Loretta Creativo looks pretty comprehensive, thank you! I'll add it to my list to research further.
@Aged Pixel wow that is a slick e-commerce site! Is the shopping cart using FAA's cart? The shopping stays on your site with your themes, so there's no jarring change of scenery when switching to buying mode. Would you have links to those premium themes which have native FAA support? I tried googling yesterday and just now, and can't find anything. I get that I can use an iframe, but that results in having to jump to pixels.com for shopping which is not ideal.