Let me start out by saying - no one really knows what the D&D Fourth Edition sales are except those people with direct access to the numbers backed up with internal knowledge of what those numbers mean. Everyone who makes pronouncements based on anecdotal evidence usually has their own cutlery to grind, either in favor or against it.
To this I can add only my own anecdote (and bundle of knives) - Half-Price Test.
I love to browse used bookstores. They are a place for both knowledge that has been passed on as well as knowledge that has been abandoned. I love finding some arcane reference work or history that has been out of print for years. But in addition to volumes that have been cluttering up the attic, used book stores are dumping grounds on a personal level (books you have received and don't want) and corporate level (overstocks and over-orders that can be unloaded for less than it would take to pulp them). The latter types of books show up particularly the larger chains, like Half-Price Books.
So with that in mind, a good register for the sales of 4E should be the Half-Price bookstore about a mile west of WotC itself, in Tukwilla. If books are being dumped, either by individuals or companies, we should see something there.
And... I don't. I come back in on occasion and find a lot still from the 3E glut, from WotC and other publishers, but almost nothing on 4E. This could be the result of three things (all of them good for WotC and the hobby in general).
1) 4E is selling and sticking. People are picking up the books and keeping them (and extending the assumption, playing them).
2) WotC is keeping a sharp eye on print runs. TSR (and WotC) both had a habit of printing more than they needed, just in case. With returnables and just-in-time delivery, there has usually been a lot of copies floating around in this gray area. I'm not seeing it. They may not be overprinting, and that conservatism carries into this tertiary market.
3) 4E IS coming into the Half-Price market, but has a very short shelf life, which would make the Half-Prices of the world very happy, since THAT part of the shelving is earning its keep very nicely. It also means that 4E has an additional interest among the cheapskates of the world (like, um, me).
Now, this is one more little bit anecdotal evidence, based on the Seattle area stores, which SHOULD see a solid response (when TSR was in trouble, there was a proverbial flood of brand-new TSR product in the Milwaukee stores). I'd be interested in hearing other tertiary market stories, and seeing if this bears out.
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