If you've ever bought an album that includes the remainders of a collection that has been stripped of its best items you have purchased what the stamp dealer considers trash. Of course, as you are getting a number of stamps at a good discount from catalog value (known as album remainders) this can prove to be the most economical way to add stamps to your collection.
Zeppelin Commemoratives Rise From The Bargain Box
The Akron-Macon commemorative covers those event covers with cachets honoring the U.S.'s lost airships of the 30s. After a definitive catalog listing the items was published, these items, which were gaining in popularity in the 80s rose in value. Suddenly the zepp memorial covers that had languished in dealers' bargain boxes at prices from $2 to $5 were going for $25 and up. Yet another example of philatelic literature providing a legitimization of philatelic items.
Flight covers from 40s, 50s. These airmail covers tracing the opening of air mail routes, domestic and international, were 25 cent items not so long ago. Today you can find them for sale anywhere in the $2.50 to $5.00 range. Rarer ones, of which there were fewer than 100 carried on a flight have risen in price correspondingly, although demand truly isn't as great as it once was when collectors counted among their numbers more of those who lived through the pioneering period of airmail.
Sand Dunes, stamps from the Arabian Gulf, not so kindly also referred to as wallpaper, so inexpensive you could paper your walls with them more cheaply than, well, wallpaper, these issued have gained a measure of respect with interest in their geographic and political area. Still, many serious philatelists still consider it junk.
Changes in the world economy obviously has an effect on the prices and popularity of stamps. It wasn't so long ago that Chinese stamps were affordable and simple to acquire in reasonably priced lots. Now prices are up in some cases 20 times what they were five years ago.
The Unusual Often Unfairly Considered Philatelic Trash
Anything that isn't a stamp or cover often gets nearly ignored by stamp dealers. Especially the smaller dealers don't have the time or the personnel to chase down values of philatelic items that aren't stamps or covers. In fact, in pre-Internet days many didn't know how to price covers, as with no definitive work to deal with everything that's out there, as there is with a worldwide stamp catalog, the effort usually didn't pay. Specific catalogs for varieties like stampless, airmail, classic U.S. etc. exist, but the small time dealer of the type found on eBay does not have the finances to put together a reference library of the scope that the bigger dealers and auction houses depend upon to price their material correctly.
In short, with unusual philatelic items - presentation cards, unusual cacheted covers, privately issued stamps and labels (aka cinderellas) the collector willing to do a bit of legwork can come up with discoveries in a box lot of miscellaneous philatelic material that can allow him to sell a few items to make his money back while being able to add other items from the lot to his collection.
Sometimes One Digs Through Stamp Trash to Find Their Treasure
Although "Mystery Box Lots" are offered in philatelic print publications and on the Internet, these have been put together from material that has been filtered down from dealer to dealer to dealer. It's better to get such lots from off the beaten trail, whatever that might mean to you as a stamp collector, whether it is a little stamp shop in a small town, another collector, or an online stamp trading site that does not enjoy the number of buyers and sellers that eBay does. In the online auction world, a miscellaneous lot you might get for a bargain under other circumstances can often be bid up to a price that will not make it cost effective for you to purchase.
The worst scenario is in an online auction situation where the lot contains a number of items of a particular topic - trains, for example - that the train topicalist finds irresistible. When a few topicalists find such an item and each decides he must have it, the best thing one can do is get out of the way and let the bidding war wage without you. Unless, of course the lot contains items of your topic. In which case, good luck.
Some items like U.S. Classics, Western Europe, pre-1940s first day covers, early Switzerland, German States, pioneer airposts, etc., will always be right. And though some people give any area the cold shoulder ("Oh, that junk...") there will be enough people showing interest to keep demand high and prices stable. All trash will not always be trash, but be assured that all recognized classics will never be less than classic.