Great question, and upon reflection (I actually looked this up in my PhD dissertation just now!) I agree. I actually can't remember the last time Bethany and I used a joint purchase auction. For some reason it never comes up -- we just each buy things and don't worry about joint ownership. If we did disagree about whether to buy a household item we'd probably just straight up yootle for whether to buy it (with the cost split 50/50 if we did).