Just like Jello is a brand name and so is Kleenex. If the person posting that photo was selling it and not disclosing it, that's one thing. But they were just showing a pic of their bass. That's all. Not any different than the many Bach or Epiphone Thunderbird owners who have pimped or modded their birds to look like Gibsons and have posted photos of them on the internet without a list of disclosures and caveats.
I don't think your analogy holds. Jello is used to mean gelatin, Kleenex for tissue, but "Les Paul" is not a substitute term for guitar. On the flip side, when someone says Kleenex, you don't think of Kimberly-Clark Corporation, but when someone says Les Paul, that name is identified with Gibson and vice versa.
I've seen guys post pics of their modded Bachbirds without saying that it's not a real Gibson, but only in Bachbird-related threads, not elsewhere. I haven't seen anyone trying to mislead anyone about it.
It has nothing to do with whether or not you're selling. It's a matter of being ethical, and IMHO by saying nothing, this guy isn't.