…a poster who never posted and don’t know what their userid is?
Is it possible?
If you just want to get a look at their profile, and you at least have their username, you can use an url like this:
http://boards.straightdope.com/sdmb/member.php?action=getinfo&username=foo+bar+quux
Replace the “foo+bar+quux” with the actual username, of course, with any spaces in the name converted to a “+”.