Originally Posted by Halk
I don't mean to sound dismissive, but you're only talking about a pure monopoly, which is in the realm of economics textbooks.
The thing I usually look for is having at least three* market players who are sufficiently competitive to matter. Book-selling, even on-line selling of PoD books seems to have that: Amazon, B&N's marketplace, plus the aggregators like ABEBooks (and similar businesses).
*Works best when at least three are individually significant, but you can get away with filling the third slot with a whole mess of smaller players (considered as a group) as long as the smaller players are relatively easy for customers to find. Or so sayeth the economics and public policy papers I've read. Your mileage may vary -- remember econ is the dismal science, and public policy is worse than that!
P.S. Of course the "three players" rule of thumb assumes that the players actually compete
, rather than trying to do the oligopoly thing. In theory the U.S. Justice department and the Federal Trade Commission are supposed to keep track of that. Further, deponent sayeth not.