The intermediary offers the ability to transact right away. If the seller thinks this is worth the discount then they'll sell and that's a win for them. If the

The intermediary offers the ability to transact right away. If the seller thinks this is worth the discount then they'll sell and that's a win for them. If the buyer thinks that's worth the premium then they'll buy and that's a win for them. The intermediary gets paid so that's a win as well. Getting too deep into the stock market/NBBO analogy breaks down because houses aren't undifferentiated fungible assets and so the details don't line up.