The "legal side" of the post is very hard to address since a) without more info we really can't size up the +/- of a legal challenge; and, b) we really don't "do legal" here very well as we're mostly non-legal and the TOS says were shouldn't anyway.
That said, the more generic the subject is, IMHO the less likely you will be able to stake out an exclusive claim to the domain name. IF you Google the root words of the directory name do you get more than a few exact match hits? (Other than links from your own website?) For example: "business directory" = 36,000,000 hits. Hard to argue for exclusive rights to use business directory in a website address. "Doofus directory" = 0 hits. You might want to lay claim to that . . . or not. ;)
It's a fairly complex analysis, to address the particulars of the legal approach, so the best we might offer is to investigate a purchase.