Wholesalers have agreements with retail brokers to provide a certain level of "execution quality", and that's what they compete on (fiercely). If they pushed th

Wholesalers have agreements with retail brokers to provide a certain level of "execution quality", and that's what they compete on (fiercely). If they pushed the price around on one order, they'd have to make it up on some other order to meet their obligations. It's measured against NBBO at order receipt time, so chollida1's example would count as negative price improvement (even if the best offer was only showing 100 shares and not 2000).