Forum Rule #13 states you shouldn't post your own car for sale. A seeming loophole is a particularly special SVT or Contour, for example the supercharged one on eBay, one with 20K miles, or something of that nature -- but note that these aren't actually posted by their owners and are consequently not "classifieds" posts.
Advertising in your signature is perfectly acceptable and in fact recommended by CEGers since the Classifieds section these days is teeming with scammers. It also catches the attention of people who would not be otherwise browsing the Classifieds specifically for whatever item it is (which is often the case for miscellany like Apple Airport base stations or floor mats). Feel free to put a link to the eBay auction in your sig.