Well as someone that frequently posts such things on NCWW (although Martin is becoming even more prolific), I can say that I have had contact with someone asking me about whether this was sold and did I have other things I was selling. And that was with the preview of "Not mine, not endorsing..." and the link to the CL post included. It's understandable that we jump right to the pic of the shiny new thing and not read all the text in the first line.
So is it necessary - legally or otherwise? Who knows? My typing has improved enough that it doesn't take much time and it MAY prevent misunderstanding. Not every user here is tuned in to the subtleties of which forum this was posted in.