I've just (read as: 'finally') released an Installation Handbook for the YAK shopping cart plugin. It covers all the steps in installation and configuration, including PayPal and Google Checkout integration (although the latter is still a work in progress), downloadable products and products with multiple types.
Hopefully this resolves one of the main complaints about YAK (the lack of comprehensive documentation). It's also an example of "eating your own dogfood", as I'm selling the Handbook using YAK.
For more info, see the YAK page: http://www.briggs.net.nz/log/projects/yak-for-wordpress or the Handbook page: http://www.briggs.net.nz/log/projects/yak-for-wordpress/handbook