Yes, most of the very rich have acquired their wealth through unfair means, be it through unfair government action like intellectual property, subsidies, bailouts, low-interest loans, etc., or violence, historic or current. Regarding pure socialism/communism, a war to fully redistribute the riches of the wealthy will likely never happen, and cause great devastation if it did.
In a libertarian system, even if everyone started out equally wealthy, some would end up richer than others, as people of differening competency and diligence should. But libertarianism makes things fair even if some are very wealthy.
One who owns a lot of resources can rent or loan it to others without much additional cost to themselves, and make a percentage return. But by making them compete with other rich people, assuming resources are plenty, the ones that offers it for the lowest price, ie., makes the closest to zero profit, will get all the customers.
As for why even partial wealth redistribution isn't preferred in a libertarian society, note that the company that provides the customer with the most value for their money wins. The riches of a wealthy person are assets to be used to finance their business, even if turned into houses or jewelery, since they can be liquidated when needed. Taking these away punishes the best company, which is counter-productive. If a lot of it is turned into stuff like parties and luxury vacations, the company isn't being very efficient, and will likely be replaced quickly in a free market. If not, that means people who can afford it are willingly transferring their money by overpaying due to their affinity for the entity, in which case libertarians would argue they have every right to spend that fairly earned money.
Besides the libertarian argument that one has the right to transfer their property to anyone they want, including their family, note that children have been raised and molded by the parent from birth, knowing that they will inherit the family business. As weird as it might seem, simply consider them a continuation of the dead parent; they will now continue administering the business. Then the same arguments as above for respecting private property apply.
By the Kickstarter model – if a high enough number of people agree to pay your desired price, you take all their money and give them all the product. Otherwise no one's money is taken and the product is never released. This latter condition is needed so people don't just wait for cheaper copycats.
The latter situation isn't a novel scenario, the company miscalculated the demand, spent resources developing a failed product, and will have to eat the losses, something that will always hapoen.
Let's say company X spends $14M to discover a much better production method, and starts using it to sell cheaper/better goods in the market. Big company Y decides to spend tens of millions to discover it themselves, and at great speed, hiring many scientists, and conducting lots of trials simultaneously. It looks like it'll take Y only 60 weeks to make the discovery, and 20 weeks to bring it to production.
Within that time, X realizes they won't be able to make $14M profit. So they decide to sell the discovery to Y for $12–14M on the condition that Y waits 40 weeks before launching the product. Y agrees, because they save time and money. During those 40 weeks, X makes $6M profit, bringing their total to $18–20M. The $4–6M is a more than excellent return for the time X spent discovering it.
Now that both X and Y have the discovery, they can either compete against each other, in which case all is good, or they can act as one and fix prices. In the latter case, big company Z decides to spend 10s of millions to discover it themselves and beat XY's pricing, in which case the story repeats.
The use of exact numbers makes it seem like this example is attempting to fool you somehow, but the only essential part is another company having the capability to rediscover the method. They won't even have to try, Y can just offer to buy it, with the implication that they will go all out trying to rediscover it unless X agrees to sell.
Thus no human effort is actually wasted discovering the same thing multiple times. Now things might not work out this way all the time, but in a free market, it will most of the time, and that's enough. Remember, the alternative we're trying to prevent is one company being able to charge whatever price they want for a potentially must-have discovery, for decades.
If the resource is replaceable, like fossil fuels, the free market will invest appropriately in alternatives as and when needed. As for irreplaceable resources, it would be a very rare situation for there to only exist a few sources of it globally, controlled by a few entities, and for them to all stop competing and fix prices. This would be a global crisis requiring international action and co-operation for diplomacy, sanctions, and acquiring new sources/technologies. Clearly not a usual scenario that can be used to dismiss libertarianism.
Today, shipping costs are so low (eg., $0.50 per kg for US-China shipments) that for most goods, the market is global.
But what if the main water supply in a region is owned by a single entity, and they charge exorbitant prices? Unlike food, bringing in water from faraway regions would be too expensive. Of course, people moving out would result in the entity going out of business and selling the water supply to someone more rational, but due to various reasons, moving out isn't always easy.
Here's the true solution. Currently, the water supply in most countries is owned by the government. Upon transition to libertarianism, the water supply in each region would simply be an entity with ownership distributed among the residents. If some of them sell their shares of the water supply to a company that then jacks up the prices, they'll have to move out, but that is simply them facing the consequences of risking putting their water supply in another's hands.
What if the water supply was sold by the government to a private entity currently subject to regulations? Would a libertarian transition remove those regulations? Note that this situation is caused by having to transition from big government to libertarianism. If a libertarian society arose naturally, rights to natural resources would be distributed among the community via the principle of homesteading. And as before, anyone selling their rights must be moving or have another source ready, othewise accept the potential consequences.
To fix the issues caused by a non-libertarian government, non-libertarian measured may be needed. The government had no right to the water, nor to sell it to the private entity. It may need to be taken back.
If someone buys up a lot of the food and medicine and tries to resell it at a high price, people will have to buy it, since unlike for non-essentials, they can't just wait for the market to make more, they need it immediately. But this can be solved easily by the community signing an agreement with the supplier before production.
Price fixing would require every single company selling something to cooperate. And if they do, another person sick of them can start a new company and undercut them. Price fixing would also destroy any goodwill towards these companies and many customers wouldn't patronize them even if they lowered their prices again.
When a company begins selling a product, the price starts off higher to recoup the cost of development, advertising, equipment, land, etc. Some costs don't increase proportionately to the number of units sold, so the cost of making each unit goes down as more are made. Thus a new company entering the market will have to charge higher prices. Does this mean that the existing companies will be able to get away with charging high prices forever because the barrier to entry is even higher?
Let's say that a company needs to charge a total of $100M more than their final prices to recoup initial costs. But by using investor funds to pay said costs, the new company can collect this $100M over, say 10 or more years, instead of 2 or 5, making the additional cost of each unit from the final price very small. Thus they can beat the overcharging company. Big investors are surprisingly ready buy billions worth of stock of companies that have been losing billions every year, if they believe it's a necessary sacrifice that will more than make up for it in the future.
Human desires are limitless. We can never have too much of something, so people would move into the remaining jobs and produce more of that. Workers would be paid less, but due to automation, everything would also be cheaper. This would work until the very last jobs are replaced. In a utopia where there is no need for labor, art, or science anymore, a market economy is no longer needed, but I doubt that day will come anytime soon.
A customer that can afford it may willingly overpay for a product due to brand affinity/prestige. Even otherwise, there are 8 billion people on the planet. Getting $10 profit from 100 million people means a billion dollars in profit. None of the 100 million people need to have been poor or exploited; small profits per person can result in a billionaire simply due to the large human population. This is not to say that all current billionaires obtained all their wealth through such innocent means – many have used violence or state assistance – just that it is possible.