without/before commerce one would link to similar websites as much as possible. Now those are called competitors.

I bet one could make a facinating ranking algo that groups sites by subject then sort them by nr of links to others in that group.

So the perfect SEO would be to have a blogroll at the top of the left menu with every related website in it.

i.e. 3 stores sell the same item. Nr 1 is the one linking to the other 2. Extra points for linking to that specific product page.