Acquiring in the Dark, Selling in the Light
The best way, in any event, to deal with changing valuation is to look at auction realizations for the past year or so and then add 20%. Bear in mind that in a normal market, and this market will be normal again, dealer relationships [for advice and access] are important. They aren't just sources of material. Experienced dealers provide perspective and few meaningful collections are built without help.
It's also important to understand that the downturn does not affect all material in the same way. The best material [as in condition, rarity and appeal] will decline relatively little, perhaps 5% to 15% while more common material with condition issues may fall 50%. It's also necessary to appreciate that sellers may be slow to acknowledge price declines so its logical, may I say necessary, to confirm recent auction realizations for anything over $300. You can look on the listing sites for free for asking prices but they tend to be high. Auctions are the cash market. As well, if an item frequently comes up at auction you can use the AE global search to look for it in all upcoming sales.
But now having made the case for auction records as the arbiters of price when prices are declining, let me also say that many dealers, over their careers, develop a keen sense of condition and focus on the best copies while avoiding lesser examples. To the layman two copies may look similar but one be a steak and the other a Big Mac. The difference in price can be several times and yet the best copy is almost always the better buy. When in doubt always ask questions and if uncertain, wait. Your understanding of quality will be crucial to your collecting experience and it takes some time to develop.
Coins are graded on a points scale. Books are far more subjective. If you don't yet understand condition you will invariably buy indifferent material that fools the amateur but is transparent to the professional. Buy too much of such material and you'll become afraid to sell it at auction for fear of being humiliated, one reason so much material is offered anonymously at auction or disposed of only after the collector is beyond hearing, knowing and feeling Understand condition early on or select a category, at least initially, that is not condition dependent. When you someday exit the collecting experience, if you have been tough about condition, you will do fine. This is the challenge.
All this said, the recent decline in prices presents, I believe, an unusual opportunity for collecting. We have probably rolled back prices a dozen years and now know much more about day-to-day availability and relative condition than we knew back then. Millions of items are continuously accessible in single searches and tens of thousands posted fresh every day. As well, the scale of material available makes intensive narrow collecting a possibility for the first time. In fact there has not been a comparable collecting opportunity since the 1950s.