Perhaps the death of Gary Gygax had people wondering about their old D&D books. We took in a couple collections recently, including a huge collection of 1st and 2nd edition D&D modules. "Modules" are what they used to call adventures, for you young-ins. All the display cases are filled with modules right now, or at least they were when I left last night. Customers were snapping them up from me as I was pricing them. To give you an idea of collectibility, a used Player's Handbook from 1st edition, unless it's a first printing, is worth about $5. A good condition Tomb of Horrors, one of the earliest 1st edition adventures by Gary Gygax, goes for $35. Adventures sell poorly when they're new, but this is reflected back in their rarity and collectibility.
Why do adventures sell poorly? Lets compare it to a popular supplement book. Say you've got a new release of Complete Left Handed Adventurers. This long awaited tome of adventuring knowledge finally satisfies the southpaw demand for new left handed feats, spells and prestige classes. Even better, with a few complex charts and some Java programming, you can convert this ground breaking book for right handed adventurers, which means everyone can use it. The players can use it, and the dungeon master can even make left handed non player characters, or given a few hours of conversion, right handed NPCs. That's 100% of your D&D audience interested in CLHA.
Now lets look at an adventure. Left Handed Gully Dwarves in the Dunjon of Celestial Pain. I made this up using a handy adventure name generator. Unlike a supplement that appeals to players and a dungeon master, this adventure may appeal to the dungeon master. Players who read adventures are to be bent over a brazier and impaled by a portcullis (sorry, a reference from an earlier post). Assuming an average game group has five players (including the DM), you've just narrowed your market down to 20% (although DM's buy a disproportionate amount of products).
It gets even narrower. Left Handed Gully Dwarves in the Dunjon of Celestial Pain is a 1st-3rd level adventure. In reality, this adventure is for 1st level characters, that will take them through to third level. For the sake of argument, lets assume that it works for levels 1-3, which is roughly 15% of where you can be on the 1-20 level scale. This isn't entirely accurate because there are a few "sweet spots" for adventures, one being low level adventures like this one, when games first start (most games die later on, unfortunately). The other sweet spot is around 5th-7th level.
Back to the math, you have 20% of your customer base potentially interested in buying a product that will be useful to them 15% of the time, and then only once. Complete Left Handed Adventurers can be used over and over again, while a game group can only use that module once, ever. As a product, it has re-usability only slightly higher than food. So 15% (the adventure spread) of 20% (percentage of DM's) is 3% of your customer base. When I say customer base, I mean my role-playing customer base, which is 15% of my customers, of which 60% play Dungeons & Dragons (a total of 9% of my customers). You don't see me sending out postcards to the .27% of customers I think might like this product.
From a store perspective, my average Complete book will "turn" or sell around 6 copies a year. That's fantastic on a scale where four turns is great. A WOTC adventure will turn maybe two times (acceptable, but not good) while a third party adventure will turn one time or even less (poor). One turn is in the realm of community service. It's no longer product, it's a decoration with a price tag. I've heard it said that one reason for the D20 license was to allow other companies to come in and provide the products like these, the 3% products that a large company can't be bothered with.
So there you have it, nearly impossible to sell up front, worth a small fortune in 30 years. Come snap up some 3.5 modules, I mean 30-year bonds, while supplies last. If you believe that, I've got a mint copy of Complete Left Handed Adventurers I would like to sell you.