

OMG, dude, that much salt is going to get your blood pressure up. You seem to have an extremely naive and tenuous grasp of economics, but OK, let's address your "arguments", such as they are:
"You are free to start your own business."
Barriers to entry much? I'm going to go ahead and guess that you've never actually started, much less run, a successful business, let alone one in a market with two behemoth corporations that effectively constitute a duopoly. Incidentally, I founded the first 100% broadband end-user ISP in America in the mid-1990s, and ran my own technology consultancy for over a decade. This is an area in which I have substantial expert knowledge.
Reverb itself was founded in 2013 by billionaire software executive David Kalt, who purchased Chicago Music Exchange as a vanity business after selling his options trading company to Charles Schwab in 2011 for $1B USD. That's the kind of capital it takes to enter a market dominated by eBay, and undercut their prices while offering demonstrably fewer options to the user.
Many of us flocked to Reverb only because eBay was so egregiously craptastic to their sellers. As with all monopolists, now that Reverb has successfully divided the marketplace, they are flexing their muscle exactly the same way eBay did. They know that in the short term, no one is going to offer any real competition to them.
"We are such an entitled bunch of whiners."
OK, Karen. OK, Boomer.
"Reverb is some kind of social service that should have their profits capped."
Strawman much? Literally no one said any such thing.
"Be grateful that they are still providing you the best service available."
That sounds an awful lot like "shut up, or I'll give you something to *really* cry about". Creeptastic, and demonstrably incorrect. They are not, in fact, "providing the best service available", when they are arbitrarily raising prices by 42.8% in one blow.
There have been no substantial changes to market conditions that could possibly warrant such a price increase. This is nothing more than a naked money grab enabled by the fact that Reverb and eBay effectively constitute a duopoly in a captive market. In fact, market conditions, by any sane stretch of the imagination, demand a price *decrease*, in order to incentivise more business. This is Econ 101, supply and demand basics.
What is obviously going on here is that Reverb's sales figures have taken a gigantic hit due to the fact that, thanks to the global economy being shut down by about 50% (or more), comparatively fewer people have the cash to buy non-essential goods, and quite a few of us are desperate to sell to make rent and utility payments. It's a move that clearly their customers dislike, and that's not a good sign for Reverb's future profitability.
Monopoly power is not "capitalism", it's a *failure* of capitalism. But, hey, keep licking that boot for more salt.