If it’s not one thing, it’s another for Craigslist, which seems to be turning into the whipping boy of the classified advertising industry. The latest indignity: a Texas real estate company First Call Properties, is suing Craigslist for trademark infringement based on ads posted by users.

First Call alleges that, shortly before it began advertising on Craigslist’s Corpus Christi site in March, rival AAA Apartment Locating began posting Craigslist ads that use the phrases “first call,” “call first,” and “call us first.”

First Call alleges that those ads were placed in a deliberate attempt to confuse consumers. The company also asserts that Craigslist knew that AAA was using First Calls catch phrases and did nothing to stop the ads.

The lawsuit was filed in May, but jumped to attention when it was transferred to federal court in the southern district of Texas late last week. A state court judge has issued a preliminary order banning Craigslist and AAA from using “Call First,” “First Call Properties,” or “Call Us First,” or similar terms.

The direction of the lawsuit is unclear. Federal law says that some publishers are not liable for damages when they run paid ads with copy that infringes on trademark. But the ads in this case aren’t paid – Craigslist doesn’t charge for real estate ads in Texas.

On top of all this, First Call also claims the ads are libelous. They have a point there: one ad allegedly said “First Call Properties is a Scam.”  But as we’ve seen in the past, Web sites in general can’t be held liable for content created by their users.