There is no legal maximum for the percentage real estate agents can take, in the US. Rates are also not fixed, by law, and are required to be negotiable. There's a general standard for rates (typically 5-6%, split between agents/brokers), but there's nothing stopping them from setting it to 99%, other than the fact that people won't pay it.
Source: was a licensed real estate agent for a long time.
Source: was a licensed real estate agent for a long time.