Hi 753C,
Thanks for your response. Since it is in your line of work I would assume that you already know more than I can contribute. However, I'll offer the following (and completely deplete what slim knowledge I have).
1. Recently I came across two books on the shelves at Borders and bought one of them:
Official Price Guide to Collecting Books - 6th Ed by Telford and Goudey.
It provides general advice and guidance for book collecting plus listings of selected prices for rare editions. So it doesn't have every book one might think of. But one can find, for example, that a first edition of Tony Morrison's Beloved (1987) goes for $300 - $500; or Beckett's Waiting for Godot (1954) for $1500 - $2000.
But no Gutenberg.
ps: It also includes a few links I haven't heard of, but which I can't find now that I look for them.
2. A used booskseller we visited recently suggested bookfinder.com as much more comprehensive than abebooks, but we haven't looked into it in detail. Bartleby also comes to mind, although I haven't used them for search.
3. There must be antiquarian news sources that report auction prices for the much rarer items, but I suppose you are already on their trail.
4. I'd appreciate hearing what sources you look to, since I am getting interested in the topic also.
Cheers and Good luck :flowers: