Here's a real-life situation involving myself and one of my wife's adult kids (....yes, I've been married more than once, I happen to like the taste of wedding cake, don't judge....)
They need a car because their other car was totaled by someone running a red light, and they finally got the insurance payout. They know I know about cars (I buy recreationally from the auto auctions, do repairs, etc). When they were looking, I told them to beware of vehicles where the seller doesn't/won't provide a vin, and when it's got a rebuilt/reconstructed title and the seller indicates it's from something exceptionally minor. All the cool kids want to buy cars via FB Marketplace, where every hack and scammer also want to sell, and of course these sellers are 'really chill and really friendly and nice!'. I've saved their butts from buying cleaned-up disasters a couple times. Here's the last exchange (they now have a vehicle)
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I know that some of the vehicles they were looking at were from either scammers or curbstoners flipping trashed vehicles that shouldn't be on the road (most of the time no inspections are done on rebuilt vehicles - just simply sign a statement that it's good-to-go). I'd also let them know about some vehicles that were sold for pennies on the dollar at the auction. And provided before-pics of some vehicles.
1) Do I let them know about some 'dealers' they need to be wary of?
2) Do I point out that they're getting ripped off, and maybe advise them to wait a bit to see what else turns up?
or
3) Do I say "well, you're an adult, this is the real world and people can buy/sell whatever they want, and since you're younger and don't have my experience - tough luck, life's a bitch, hope the roll of the dice turns out in your favor"
Most of this is rhetorical. Because this is family - I would want to make sure they are not taken advantage of. But I'd also feel the same about my good neighbor. But not the bad neighbor, they can rot in hell
.
To me PCF is more of a community than an open classifieds marketplace, and a quick point-out (to me) would not be unreasonable. But I'm also going to say that I don't know if there's going to be a correct answer. Way I still see it, is what's good for one is good for the other. Gonna blatantly try to excessively profit at the cost of community? Be prepared for someone to point out what you paid for those items last week/month.