One thing I learned a long time ago in a different collecting hobby is that it is REALLY EASY to find out what is rare and desirable. Just look for what you DON'T SEE AND NEVER SEE on eBay. That is the stuff that is rare. It never makes it to eBay. These are the train show transactions that never make it out of the parking lot--the ones where the guy can't even get his car unloaded. These are the antique transactions from estate sales that take place from one estate liquidator to another or to an auction house long before the estate sale is ever made "public".
Desirable is discovered by checking professional, off-line auction results (not eBay). When the auctioneer's catalog projected the selling price range as $6,500-$9,200 and the final bid was $15,900, you are getting a good measurement of "desirability" of that item in that condition and possibly in that region or country.
I'm no expert. But close observation will usually reveal what we want to know.
Don
