Hmmm. This seems like an odd way of engaging a broker to sell your domain names.
Typically, a broker (regardless of industry) will be very selective in the products they wish to represent. They choose the products based upon a rating system they adopt, which indicates the likelihood of a successful transaction. When they engage a client agreement it will be based on a high commission fee for a sale successfully completed.
Now keep in mind that I said typically, which doesn’t represent all relationships in the seller/broker dynamic. Some high-volume brokers (non-industry specific) will charge a basic fee up front to cover administration fees. Usually these brokers, provided they are legitimate, will also provide full transparency on their fees/services and will actively work to produce some measurable results.
It depends on what is brought to the table from both the asset holder and broker. It depends on the terms of services agreement (promises/guarantees etc..).
Some brokers could be well worth the money up front, but as a seller I would imagine that you need to go into the potential deal knowing the true value of the asset(s) for sale, and with no uncertainty on the terms of the agreement. As with all business dealings the more informed we are, the better our expectations can be managed.