Flippa for one demands that if you're selling in Flippa you can't list elsewhere.
How do they know if you do list elsewhere?
I find today that sites like tvfilmrights.com, PPMLogix and others (I won't do Flippa's homework for them) are listed at both Flippa and websitebrokers.com. I find duplicates of Flippa listings elsewhere, with other brokers and with classified listing sites such as bizquest.
Is exclusivity just a scare tactic or does Flippa do any checking? If they don't then really you should be listing in multiple locations to maximise your chances of success and minimising your liability for success fees (or would you still have to pay Flippa success fees if you sell your site at a broker?)