While I can agree somewhat with impulse buys, I do think one of the issues with buying from a member here is that 1) you need to be an actual member with a minimum post count, and 2) the prevalent notion of sellers is not "buyer beware", but is instead "buyer satisfaction" reflecting the modus operandi of our beloved cottage industry vendors. We don't see it much - I presume because its handled through PM - but if there was an issue with the product, I'm sure the individual seller would do what is reasonable to do to ensure the buyer is satisfied. Otherwise, the unsatisfied buyer is welcome to post their experience here. In my opinion, there is value in that as opposed to places where we might be able to purchase items more cheaply.
You might want to check the thread I posted a while back as I questioned why used gear was often priced very close to what you could buy it for new. I learned a bit from the responses. I also learned a bit from selling used gear myself and incurring the costs of shipping - which has gone way up since I last sold something online.
Not saying that there aren't times when something is posted for sale for what can reasonably be said is "too high" a price. But in general, their are other things to consider which deserve respect. It may be semantics, but I think this is what folks have referred to as "rude" is referring to being disrespectful of what you may or may not know about the item. To make a simple assessment based on price only is disrespectful in this regard. That said, information about availability at lower prices IS valuable, just that the individual thread for the item for sale is not the place to post this info for above reasons.