Don't say "I want to buy Example.com" or "I'm interested in buying Example.com". Say "I offer $$,$$$ for Example.com".
Send that message through the pipeline. IF it's good money for the domain you are more likely to get a message back.
If it's parked send that message to the domain parking company.
Lastly, since the larger domainers tend to hang out together and visit a limited variety of forums you might simply post your offer to buy at a few of the "other" cough cough domain forums - the ones that allow you to post specific domain names.
If you don't get a response you may simply be dealing with a domainer that isn't interested in "dealing retail". Anyone with a substantial portfolio, who deploys "domain privacy", is sending a pretty clear message: "I'm not interested. Go away!"
[edited by: Webwork at 5:46 pm (utc) on June 21, 2008]