The old adage “You get what you pay for” may come into play here.
If I negotiate and low-ball the agent’s fee, they are ethicallly free to consider me as at the bottom of their client list and work for my interests whenever they can get to it and/or choose to.
Yes, if they except my offer, they’d be bound to provide services but, as I see it, the intensity of their effort may be directly proportional my to willingness to bid for their service.
I once put in the selling contract for an additional $1000 to the buyer commission, worked well.