it might be better if you just asked the question that your driving toward. Would it be in the broker's self interest to widen the spread ? Brokers sell your flow to a consolidator who's rebate to your broker does have a spread component in it, but trades are still done subject to a market like and a NBBO - National Best Bid Offer which can't be violated(rare exceptions). What if I'm trading very ill-liquid instrument where there is exchange/dealer competition ? Then there is a disseminated quote - take it or leave it. Are there brokers who don't/won't sell flow ? Yes,but I doubt it's relevant. All this get disclosed by the broker's quarterly. The rebate/payment deals are public in US markets. It is in your broker's best interest to get your trade done at NBBO or better and then to move on.