As a 1-person brokerage, Maven Domains, based in the UK I just don't acknowledge most of the points raised in the comments above. It may well be that larger brokerages operate in the manner that has been suggested above but I can guarantee you that as a very pro-active broker my work is very time-consuming and laborious, and in a lot of cases can give me zero return on my days, weeks and months of effort.
Plus, I don't have the luxury of a list of potential buyers I can contact about buying a domain I am brokering.
I have to balance the amount of time and effort I will need to put into researching and contacting potential buyers with what I believe is the '%' chance of that domain selling at the Asking Price.
As a broker if you want me to try and sell your domain for $100k and the domain market shows similar domains selling for an average of $50k then why would I want to spend all my time on effort doing all the research to identify companies, then identify decision makers, and then contacting those people multiple times to try and make a sales case for a domain that is clearly over-priced?
I need to be able to build a sales case for every domain I broker, and one part of my sales pitch is to use comparable domain sales, because I can guarantee you that I will be asked to justify the Asking Price at some part of the sales process. And although as part of my sales pitch I will build in 'value' into why the company should purchase this domain, when you have strong comparable data then it is hard for a potential buyer to argue hard sales data facts.
And most brokers will be very selective about the domains they broker as we work on commission only. Not every domain I broker will sell, for a variety of reasons, but I will broker domains that are valued from a few $1,000 upwards. At the moment I have a LLLL.org/.net domain I am trying to sell for a client and if successful I only get a few $100 commission.
I am also brokering a few LL.com and LLL.com domains to end users, and these are the most difficult to find an end user buyer for. You should try approaching a company 'cold' and ask them to purchase a domain for $1m+ or $100k+. For some domains I may even spend money advertising on Facebook and LinkedIn and sending out a Press Release about a domain for sale. There may well be a nice commission 'if' the domain sells, but there will also be many days and weeks of hard work completely wasted if I cannot find a buyer for these LL.com and LLL.com domains. And the same is true for any decent 1-word or 2-word domain.
The bottom-line is I do brokering the hard way i.e. I pro-actively research and contact potential end-users, but I will balance that by being selective. And by 'selective' I mean that apart from looking at the quality of the domain name I will also look at the number of potential end-user buyers to see if there are enough to give me a 'reasonable' chance of making a sale.