I can only repeat myself. Most people will finish the bike quest long before they have to care about finishing the Kanto Dex. The demand for a dex ditto is so low there would be no point in spamming trade chat day after day with "wts ditto". It makes no sense.
If someone needed a dex ditto and wrote "wtb ditto for dex" in trade chat and you would sell it to him nobody would complain.
Also let me use your logic. You said Cirstroke didn't say he was selling it for bike quest, right? He also didn't mention he was selling it for dex. Why is that? Because he is scamming.
I'm just concerned about new players, because I know what it feels like to get ripped off.