As I read the situation, which may be totally wrong, if any resentment exists, some of it may be that the long term members here are mostly vintage bicycle enthusiasts.
When a new person logs on here and posts a query regarding the value of a bike they just found, bought or own, it may appear to the enthusiast that the "newbie" is only here to glean information so that they can maximize their gain on the sale of the bike. In other words, take advantage of the knowledge of some members here in order to increase a personal monetary gain.
I've seen lots of these kinds of posts. And it is somewhat understandable that some members are resentful of someone only out to benefit themselves.
On the other hand, I haven't seen any member here be rude or inconsiderate to a new post starter who seems to be genuinely interested in learning about their bicycle and who comes here in good faith to see what they can learn about it. There's a vast vault of good information and knowledge here among members, who are mostly willing to share that knowledge.
Knowledge is power, they say. And knowledge is hard earned. What I've experienced here for the most part are members who are generous in sharing what they know and not lording it over those looking to learn. Or bludgeoning a new person with a sense of superiority, or what I call "the wise old man" syndrome. Thankfully pretty rare here. Which is why I came here instead of other vintage bike forums.