Undoubtedly two or three elements in the bookseller - book buyer equation are going to change. The dealer research premiums, however justified by the work, have not stood up and are not going to stand up. Enough collectors are willing to camp out on dealer research and then buy cheaper copies elsewhere to continue to undermine the premiums that research requires. These days, a dealer who prepares an elaborate explanation of an item can expect to see suspiciously similar descriptions soon posted from other dealers offering the same item. Even when competing dealers don't paraphrase the description their copy will come up in the searches nearby the well researched copy to tempt a buyer to read the better description and then buy the cheaper copy. The seller may know next to nothing but for many buyers price is everything.
I myself often buy less expensive copies. I tend to do my own research and to find material in obscure places. I've just bought ten items on eBay after buying about a hundred items on ABE over the past several months. I find I tend to know more about the material I acquire than the seller's do although this is not always the case. I do understand "frequency of appearance" as well as anyone and I know when to wait. Between eBay, the net and dealer catalogues there is a relative value scale although it is different for every book. In buying this way I'm probably just a little ahead of the curve but I speak to collectors all the time that understand the market as well or better than I do. Such collectors need less from skilled dealers. They are however a small minority.
For new collectors entering the field the careful and painstaking work that the better dealers do is essential. However, with fewer catalogues offered, the best tool for educating and encouraging collectors is increasingly in short supply; one reason there are not enough new collectors entering the field. Whether there are other ways to effectively educate the new collector remains to be seen. One thing is for sure though. The internet has changed the business more in the past ten years than it changed during the prior five hundred.