A sale and purchase transaction, particularly where sizable sums of money change hands, cannot be digitally automated. It's a people business. If you can't get personal treatment with prompt communication from a brokerage when in the process of listing your domain names, that's a bad omen. Brokerage fees for domain sales vary. It's not a well established business where the costs and profits of doing business have been refined over a long period of time into an industry-standard fee.
Fees vary as well as services.
The highest fee does not ensure the highest quality service, and the lowest fee does not necessarily mean cut-rate service. Likewise, the highest fee is not necessarily a bad deal, and the lowest is not necessarily a good deal. You will have to make a detailed inquiry about services, transaction arrangements, promotional efforts, fees, and the like to make an intelligent choice of brokers. But there is too much room for abuse. To be sure about the sale price, your best bet is a domain name appraiser.