I think the most important thing is that the private sales forum remains a safe place to buy - which is often repeated on the relevant fora.
(Safe being relative - price is the prerogative of the seller and choice of the buyer)
A polite PM is the way to go if something is amiss or misdescribed. (I’ve done it in the past and been pleased to see the listing amended)
As for the friends thing, I don’t know how much it actually happens but I wouldn’t offer a GLWS or even a ‘like’ if it wasn’t a watch I would consider buying myself - and that would include condition and price.