This is a touchy question and the answer you've you received from Alan is correct.
There is no fixed, standard, customary...commission rate or fees. Any broker/agent stating otherwise is in violation of the law.
It is up to the individual brokerage/agent to specify any commission or fees a seller or buyer, if subject to a representation agreement, exclusive or nonexclusive, shall be responsible for.
In both cases all parties must agree, in writing, to all fees or commissions.
Commissions, by law, cannot be fixed. Each agent, and agency has their own commission structure based on the services and costs for those services that they provide.