>Such as what? Workers already own property. Most people have a TV, a house (that they own fully after their mortgage matures), a car and so on. Should we steal all of that stuff too? Or just the lambourghinis and yachts of the wealthy? Sounds like "lets just steal stuff we want from other people" to me.

Marx and other 19th century writers make the distinction between private and personal property (at the time, it was simply called 'property', and personal property called 'possession'). An example of private property is a factory, machinery and land upon which people are employed to make things. These things are social means of reproduction. I don't think anybody, Communist or otherwise, is advocating for taking personal property such as cars (unless property of a limousine company), televisions, computers (unless property of a commercial software house) etc.

>my understanding is that capitalism exists to allow people to compete to be the best at providing services people want

I think that this is merely incidental, capitalism producing goods for exchange rather than usage is not incidental. Not even bosses are in charge of capital, capital is its own owner, and ensures its continuation through the capitalist system. This is why sometimes you see property owners giving themselves salaries of $1.

I don't think that people who provide the best service rise to the top; rather, those who are the most ruthless in providing that service rise to the top; it's not even proportional to the amount of labour applied. In fact, many products are made which people wouldn't buy if it wasn't for a lot of advertising. Accumulating capital is one of the goals of a property owning actor in a capitalist system, and that can be best done by paying the workers the minimum, that is, the amount required to maintain the worker and ensure the continuance of the flow of workers.

> The idea that a community can own property that is shared seems bogus when you extend the argument to things that are scarce.

I think that the community should collectively own means of production; however the labourers themselves as individuals own the product of their own labour and choose when and how to distribute it, if at all. A worker who makes social means of production (i.e MoP which multiple people use) would likely be persuaded to let the community use his machine, in exchange for a certain amount of product for example. However ideally, community projects would make sure that most members of the community who wish to produce contribute to the production of means of production.

>Marx himself posited socialism and communism as being a state that requires a post-scarcity and automated society, which we do not have.

It's not necessary that society be post-scarcity; rather, that resources be managed without introducing artificial scarcity. Full automation is not possible under capitalism, for there has to be someone to buy back the products which are producted. Capitalism isn't a way to get to the point of full automation.

>You can't calculate the value of a thing based on the labour required to create it.

The Labour Theory of Value[0] attempts to do exactly this.

> Otherwise what stops me spending years on some piece of junk and then demanding a reward?

Marx wanted to make the distinction between private labour and social labour. Social labour calculations take place within the context of society, whereas private labour is not commodified. He writes in Capital chap. 1:

"Some people might think that if the value of a commodity is determined by the quantity of labour spent on it, the more idle and unskilful the labourer, the more valuable would his commodity be, because more time would be required in its production. The labour, however, that forms the substance of value, is homogeneous human labour, expenditure of one uniform labour power. The total labour power of society, which is embodied in the sum total of the values of all commodities produced by that society, counts here as one homogeneous mass of human labour power, composed though it be of innumerable individual units. Each of these units is the same as any other, so far as it has the character of the average labour power of society, and takes effect as such; that is, so far as it requires for producing a commodity, no more time than is needed on an average, no more than is socially necessary. The labour time socially necessary is that required to produce an article under the normal conditions of production, and with the average degree of skill and intensity prevalent at the time. The introduction of power-looms into England probably reduced by one-half the labour required to weave a given quantity of yarn into cloth. The hand-loom weavers, as a matter of fact, continued to require the same time as before; but for all that, the product of one hour of their labour represented after the change only half an hour’s social labour, and consequently fell to one-half its former value."

>and then demanding a reward?

In a Socialist society, you would make things for your own usage, unless in some kind of barter or market Socialist arrangement.

>Value of a thing is determined by what it's worth to the person who wants to use it, i.e. utility pricing.

Marx makes the distinction between use-value:

"The utility of a thing makes it a use value.[4] But this utility is not a thing of air. Being limited by the physical properties of the commodity, it has no existence apart from that commodity. A commodity, such as iron, corn, or a diamond, is therefore, so far as it is a material thing, a use value, something useful. This property of a commodity is independent of the amount of labour required to appropriate its useful qualities.

When treating of use value, we always assume to be dealing with definite quantities, such as dozens of watches, yards of linen, or tons of iron. The use values of commodities furnish the material for a special study, that of the commercial knowledge of commodities.[5]

Use values become a reality only by use or consumption: they also constitute the substance of all wealth, whatever may be the social form of that wealth. In the form of society we are about to consider, they are, in addition, the material depositories of exchange value."

and Value (also exchange value):

"Let us take two commodities, e.g., corn and iron. The proportions in which they are exchangeable, whatever those proportions may be, can always be represented by an equation in which a given quantity of corn is equated to some quantity of iron: e.g., 1 quarter corn = x cwt. iron. What does this equation tell us? It tells us that in two different things – in 1 quarter of corn and x cwt. of iron, there exists in equal quantities something common to both.

[...]

This common “something” cannot be either a geometrical, a chemical, or any other natural property of commodities. Such properties claim our attention only in so far as they affect the utility of those commodities, make them use values. But the exchange of commodities is evidently an act characterised by a total abstraction from use value. Then one use value is just as good as another, provided only it be present in sufficient quantity. [...] As use values, commodities are, above all, of different qualities, but as exchange values they are merely different quantities, and consequently do not contain an atom of use value.

If then we leave out of consideration the use value of commodities, they have only one common property left, that of being products of labour. But even the product of labour itself has undergone a change in our hands. If we make abstraction from its use value, we make abstraction at the same time from the material elements and shapes that make the product a use value; we see in it no longer a table, a house, yarn, or any other useful thing. Its existence as a material thing is put out of sight. Neither can it any longer be regarded as the product of the labour of the joiner, the mason, the spinner, or of any other definite kind of productive labour.

Along with the useful qualities of the products themselves, we put out of sight both the useful character of the various kinds of labour embodied in them, and the concrete forms of that labour; there is nothing left but what is common to them all; all are reduced to one and the same sort of labour, human labour in the abstract."

Marx further writes:

"A thing can be a use value, without having value. This is the case whenever its utility to man is not due to labour. Such are air, virgin soil, natural meadows, &c. A thing can be useful, and the product of human labour, without being a commodity. Whoever directly satisfies his wants with the produce of his own labour, creates, indeed, use values, but not commodities. [...] If the thing is useless, so is the labour contained in it; the labour does not count as labour, and therefore creates no value."