The problem really is illustrated well with the speculative words and phrases you have used in your post. Why should we need to guess what is going on here? If he truly wants some sort of help or to warn us why not just say it. The rules for posting in the GB&U are clear on this. Especially look at the bolded parts below:
"3. If you have a grievance with someone, by all means proceed and post your situation. You should communicate with them FIRST and BEFORE you try posting a thread. We will do the best we can to help you. But understand, deals that have occurred on other forums, Facebook, Instagram, eBay, and other venues we have no power over. A heads up is appreciated, but be responsible in what you write, it is a permanent record of your thoughts. Your post should consist of and contain factual information, not assumptions or false allegations. Abbreviated threads like, ""be careful buying or selling to so and so..." with no subsequent details will not be allowed to stay open for discussion or debate. If you are heated, wait a day to calm down, no one is impressed by foul mouthed, disorganized posts that contained fragments of broken sentences or giant walls of text that no one reads. This is NOT Whine and Cheese. Do not make comments that are barbed or provoke other people to get angry. Cool, calm, and collected is the way to deal with disagreements."
If they guy wants help he needs to help himself by providing actual information. If he wants more eyes on his for sale thread there a better ways as you have suggested and as I obviously make use of myself.
We should not be playing guessing games in the GB&U.