@trytone - my experience, for the big ticket items, is that most dealers put them online to reach the broadest audience. (Similar I guess to vintage wine market, though I really don't dabble in that). The rare stuff in prog/early rock/post-pysch fusion is, based on my experience in the last couple years, far more evident at the WFMU show than the Austin show, but I think part of that is the market. (Neither show in my experience offers realistic pricing for these-i.e., no bargains, but at least you get to examine the record). I suppose most of these are "negotiating prices" and have seen serious buyers walk away with a rolling bag full or stuff or nothing, depending on their pocket book and their particular level of "need" for certain records.
I know some people "score" at shows and find valuable or desirable pressings cheaply, but most of the stuff that is regarded as "collectible" is a known quantity, both to dealer and buyer. I don't "collect" for the sake of it, more interested in listening to the stuff, but once we get into 4 figure records (or even high 3 figure at this point), I'm out.
The other attraction to online is that you can target what you want immediately. Of course, there are those who can spend days at a show and search through every bin. I don't have the energy for that, and eventually lose interest. I'm not buying to resell, and figure the inter-dealer trading before the show also accounts for some of the traffic in the high value stuff.