aramis erak
Legend
Considering they've bought up at least two competitors... with RPGNow having been the big oneThat's highly arguable. While there's no way to measure it that I'm aware of, I'd say that while pay-for-download role-playing game supplements might be an extremely niche market, there's certainly an argument to be made that OneBookShelf has monopoly power within it, even if they don't have an out-and-out monopoly.
they have a few notable competitors remaining...
Paizo's webstore (Which sells more than just Paizo products)
e23/warehouse 23 (SJG's web store; it also carries many non-SJG products)
Indie Press Revolution (which seems less relevant every year)
itch.io (which seems an increasingly relevant store every month).
pigames.net (Precis Intermedia Games, formerly Politically Incorrect Games)
It's worth noting that PIG has bought up a number of orphaned¹ IPs and put them up in PDF... like WEG's Masterbook line.
A number of publishers support their own webstores as well, but only their own products, or in a few cases, a partner company's. Or, like Chaosium, 3rd party licensees of their systems.
Amazon has a number of game designers releasing through Kindle. Still pretty trivial.
while it pains me to say it, the monopoly element of DTRPG is that people think it a monopoly and don't look elsewhere.
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1: orphaned games: their parent company has died/gone out of business, and the rights holder doesn't want to try again, despite small vocal intense fanbases.