Print on Demand would be brilliant.
I also think that there is reticence to buy old materials on ebay because they're either of dubious condition OR highly priced because they are in good condition.
So, print copies of reasonably priced older material could be wonderful, and I don't think it's quite fair to judge that market based upon ebay...except to say that, yes, there clearly is at least SOME market given the prices at which items sell. (WotC, or someone here could even do a search of which items had sold, how many, and for how much).
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I'll also add that the opportunity to buy "sets" of items (e.g. all the planescape materials) makes this idea even more appealing. Heck, a resurgence in AD&D might draw people to play that again...AND MAY EVEN DRAW PEOPLE TO START PLAYING THAT EDITION WHO ARE NOT PLAYING AT ALL.
Imagine, suddenly second edition D&D becomes popular again to a degree...or even if not popular, just plain available. Someone who stopped playing 10 years ago might hear about this and get a group together. Then, after playing a few games, they just might want to check out what the "current" D&D is...and buy/switch to the latest edition.
WotC says they want to bring lapsed players back...give lapsed players their old game again!
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This line of thought gave me another idea...why not advertise 4e in the back of the print on demand books? I'm not a huge fan of this idea, but what about, even better, including a pamphlet, super rules lite version of 4e along with sales? Something along the lines of "here's your x edition product, if you're curious about 4e, here's a taste".
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One final thought. As has been suggested in other threads, WotC maybe, potentially, could go to "pure" digital format...no print books. What if they did so for 5e, but also provided new 5e print on demand books?
And to follow that to it's logical conclusion...what if they released their ENTIRE catalog, from 1974 to present, all through PoD? Could/would they make good money off of the current and future edtions?