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1) Traffic domain buyers are currently the most active sector. If you have traffic stats then the best place to sell is in the 3 major domain forums: DomainState, DNForum and NamePros. You can also be expected to receive direct contacts. Buyers can be found paying 3-9+ years revenue, depending on the nature and source of the traffic.
2) If the domains are merely "catchy names" (no stats) then the most active market is resellers, whom you'll find in the same forums. They will pay, on average, 10% of what they think an enduser will pay.
3) Real end user buyers will find you directly, upwards of 99.5% of the time. They check the WhoIs and email or telephone you.
4) Listing domains for sale in forums or "domain listing services", in hopes of drawing in an end user buyer, is a failed hope IMHO. Better to simply set up a splash page with "this domain for sale" on it OR make an entry into the WhoIs about "domain for sale". That way the curious are encouraged to contact you and you won't incur an obligation to pay a broker a fee. Of course, if the broker is providing value added services for their fee - escrow, traffic monetarization - there is a basis for considering a "passive broker". (Passive = splash page, which you can make yourself.)
5) You could create an AdSense minisite and have a conspicuous "for sale" sign (I think, check the AdSense TOS and let me know if I'm wrong). This approach would likely be the best as it would a) make a few bucks; b) establish some traffic and build traffic stats; and, c) expose/confirm the domain as "for sale".
You don't need a brokerage service, especially one that binds you to an exclusive deal. An exception might arise for a megadomain, where you know there will be significant market interest and the value of the domain is "up there". In such a case a well qualified broker might be a proactive agent, contacting likely interested parties, validating the domains value, etc.
Entirely passive brokerages are not worth a damn in my book. "Listing" your domain for sale, in a passive brokerage site, puts you in a worse position than simply putting up your own splash page.