Mike, it is always interesting to get your perspective and tap your expertise. I've only been doing this for four years, so I have a lot to learn.
I have found it effective to watch a lot of auctions, in the brands I like, a somewhat narrow group of Briths makers (James Upshall, Ashton, GBD, Ferndown, etc.). The upside to Ebay is the sheer volume. There are definitely price fluctuations, based on many factors and certain sellers (Pipestud, Coopersark, etc.) usually command premium prices. But as I mentally scan thru the daily pool of closed auctions I do get some decent pricing perspective and can quite frequently determine to a fairly close degree where an auction will end. I really enjoy watching those last seconds tick away and see if my guess was close. Reviewing the pictures, description, vendor, time the auction ends are all factors in when a particular pipe will bring. Perhaps that isn't a "Blue Book" value, but just what the market will bear.
I believe with a some pipes on your site, the price set is by the seller. I assume you give them some counseling in getting in the right ballpark. I watched with interest last week on a GBD 9676 Virgin Colossus you had on Ebay. (I have the same pipe). I thought the initial listing was a bit high and then it was reduced and sold (I had predicted around $250 for this unusual GBD, pretty close I think?). I really was hoping that first price point would be a hit! For that pipe, it was pretty much a group of one, and I assume there was no other piece to compare or set pricing.
You are right in that online vendors will offer more consistent price point, but the pool of available pipes in a particular brand/shape/finish is much smaller.
My pricing analysis may not work with the real high end pieces and perhaps just useful in the mid-priced estates. ($100-$300)