Intermediaries come in all shapes and sizes. Some of them are a licensed agent themselves, but often include properties from other realtors and agencies in their own offering. Intermediaries can make use of the license of the realtors they work with, they are not required to be a licensed realtor themselves.
The major advantages of an intermediary are the broader collection of properties on sale and the comfort of dealing with someone who speaks your own language. Most intermediaries will offer a service where they guide you all the way, going from the earliest stage (search) to the settling in. If you contact a French real estate agency they might refer you to an intermediary themselves in order to smoothen the transaction.
Almost in all cases we have encountered, you will not pay more when an intermediary is used. Usually intermediaries have agreements with their realtors where they split the realtors commission. So unless you involve an outside intermediary yourself after you visited a specific property, you should not expect to be paying more.