Coombzy, that comment was intended as a joke that illustrated common sense regarding people you encounter online and the caution one should observe. If you want to think of this as a community, fine, but in a physical community, there is not anywhere near the anonymity that exists on an internet forum.
You go ahead and help people out. Remember that when you help out one of two people who are discussing a trade or purchase, you are also opposing the one you are not helping. So it isn't as simple as "I will be helpful and nice", since someone is the getting the business end of your assistance.
I hope everyone here remembers this discussion when they go to sell something and some unconcerned third party starts shouting out what a crook you are to want a third of what you paid for something.
The question was about whether it is rude to intrude on someone's classified to announce that you think the price should be lower. Well of course it is, even if you are right. It is completely within your rights to do this rude thing, since the seller is electing to place a classified on a forum, but it is still not very considerate of the seller. There are all sorts of reasons to ask for one price or another. If you don't want to buy something because you think it is over priced, don't. If you want to warn your friend who has asked you for advice, fine. But if you have no interest in an exchange, it is intrusive to proffer an opinion. What is worth $5 to me might well be worth $10 to you. Prices of used equipment are personal assessments.