I disagree I believe that the farmer's market has only a minor convenience to offer, and has little ability to gatekeep an individual seller from the market. That is not the case with DriveThruRPG.
The difference being that if you ignore the farmer's market, you can set up a stall a few feet away for marginal time and trouble and reap near-identical rewards. If you want to set up your own sales website for pay-for-download RPG products, you not only need much greater technical skills, but you'll also be unable to effectively reach most of your customers.
DriveThruRPG has created a barrier to entry simply by virtue of its overall success, in other words. By becoming the single largest venue, it has effectively made it more difficult for alternative venues to viably compete, and so discourages publishers to use those other venues, let alone creating new ones. That's far and away more than the farmer's market can do, hence why it's a bad example.
That's a distinction without a practical difference. Either way, you need to engage with DriveThruRPG in order to engage with your potential customer pool. By demanding that publishers adhere to certain conduct guidelines, they're effectively leveraging the barriers they've created (whether intentionally or not) to exclude meaningful market access from people whose behaviors they disapprove of.
Anti-competitive practices alone are not the hallmark of a monopoly, nor are they the sole indicator of a barrier to entry. DriveThruRPG might not mean to drive other venues out of business, nor discourage new stores from opening up, but the fact of the matter is that they do simply by virtue of being so successful that they've become, for all intents and purposes, the only game in town.
When a publisher only gets 5% of their storefront sales from Paizo, and 10% from the OpenGamingStore, and the other 85% is DriveThruRPG, that's an indicator that the latter has monopoly power, and so they've achieved a de facto barrier to entry for publishers who don't meet their conduct guidelines.
This is not correct. By this definition, monopolies don't exist, since anyone who could effectively open up a sales venue, no matter how localized or how limits its scope or reach, is always "in competition" with even the largest and most omnipresent corporations. That does not truck with what we can see in the world around us.
I believe this is an inaccurate assessment of the market for pay-for-download digital RPG products, which is the market under discussion here. The small publishers for whom most/all of their products fall under that market are adversely affected by DriveThruRPG's decision to leverage the barrier to entry that they have (and they do have it, even if they didn't seek it out) against publishers who don't meet their conduct guidelines.
When you're the only ladder in town, and you say that only the people who approve of can climb the ladder, then you are effectively acting as the barrier to entry, even if you say that the fence was already there. All the more so when getting over the fence is the only way to reach the market that's beyond it. The fact of the matter is that just because your own success has made you assume greater responsibilities, doesn't mean that you not wanting to have ever assumed them to begin with is a viable reason for ignoring or otherwise shirking them.
DriveThruRPG is not what enables a business to exist, they're what enables a business to reach the customer pool that they've managed to corner. In so having cornered it, they've gained the ability to regulate who accesses it. That's a responsibility that they need to manage appropriately, with regard to not gatekeeping those publishers.
I disagree. When you get so big that you've managed to be the source of 85% of the market revenue, the analysis is no longer the same.