First of all, as someone who tried out all the other cart plugings out there, Yak is one of my favorites. Using posts as products is always a good idea.
I would like to ask you if it is possible to reuse the tables YAK creates in a multisite installation. YAK creates 11 extra tables every time the plugin is activated. I am sure there is a better way of minimizing this number by better data modelling. In multisite install, for every domain these tables get created and unfortunately it bloats the wp database.