Unlike operators, charter brokers were historically not regulated. But believe that changed. It seems some brokers do choose to get audited by a third party, Wyvern Authorized Brokers or the Certified Charter Broker program.
Below are 2 resources for these charter brokers:
http://cheq.argus.aero/BrokerRegistry/BrokerRegistry
https://www.wyvernltd.com/solutions-wyvern-broker-directory\