Of course, one person's taxes isn't going to effect the entire country. It's cumulative. In the US, the top 5% of income producers pay around 60% of the taxes. The $400 million is taxed as a capital gain. I don't know what the seller paid originally for the art. Let's say $100 million. So his $300 million profit means he'll pay 15% capital gain or $45 million, leaving aside expenses. The agent who made $50 million will pay 38% business taxes after deductions and expenses. There also may be state and other local taxes depending on where he and the agent live.
Of course, tax rates are a political decision. But it's based to a large extent on how much the treasury figures it can get in taxes from the different income groups, capital gains, etc. The more the higher income groups pay, the less the lower income groups have to pay.
(I don't know how to highlight a sentence on this iPad, so I just have to refer to your last sentence.)
No, the more the highest pay, the more the exchequer wins; the lower incomes pay whatever the politicians think they can screw from them and still collect vote payback. The tax man likes nobody; his needs are insatiable because so are voter expectations which, in turn, are pushed his way to fulfil.
The dream of low taxes for Joe Public because Mr Zillionaire is paying more is hogwash. There are not that many of them. The real problem is unreal expectation based on a conditioned belief that somebody else should be obliged to carry your own tab. It's as unreal or unworkable as setting taxation at, say, 15% for everyone. Anyway, it isn't really the high-earning individual who can be the villain, the problem can be the way that companies are able to operate; one really shouldn't confuse the company with its founder and/or main shareholder. There are rules governing company behaviour, and plenty of tiny states willing to house their nominal headquarters and give stupidly low tax rates in exchange. Think recent events in Ireland. Laws exist, but bending them is an art that reaps big rewards, and ignoring demands to collect unpaid dues is also less than an honest way of cooperating with the world.
I'm no leftie, but abuses on such a level are more than just some little guy trying to save a few bucks for his retirement by doing the odd number without paperwork. Back to that 15 % not always being the same thing when talking matters of scale and result.