Well I don't, in this thread in any case, want to get too drawn into a debate on Marxist principles. Though Marx's and Marxist thoughts on the subject of money are fine.
Not sure if this is on topic. Feel free to tell me off!
Marx's ideal economy is one with agrarian demands and industrial efficiency. He formed his ideas in 1830s and 1840s, a time when Industrial Revolution inventions were mostly about how to make the same stuff cheaper and quicker, such as with better looms and sewing machines. Gas lamps might have replaced oil lamps, but you still only want one in each room. Later inventions, by comparison, created completely new demands that did not replace something that had always existed, e.g. phones, cars, computers, etc.
If we assume demands would be static, given the exponential growth of industrial productivity, it was not a stretch to say that one day we will be able to make all the gas lamps we'd ever need. Indeed, we probably can, right now. The trouble is that we can't make enough more complicated, expensive toys for everyone, like sport cars or iPads. By the time we can make enough iPads, people would want newer, fancier gadgets, which will again be expensive and limited in quantity.
But let's go back to the 1840s and assume that no new toys would be invented. Demands now have an upper bound, and the potential of productive forces seems limitless. What do you do when you have made one gas lamp for every room in the entire world? You put one in each room. Since everyone will have enough gas lamps for all he needs, it's no longer necessary to decide who gets the next lamp and who doesn't. At this point, money becomes pointless for consumers. In fact, the only purpose it can serve is to stop someone from getting his lamp, which is wrong and immoral when there is no other, more physical reason why he shouldn't. That is what Marx thought capitalism was doing, by not paying workers the full worth of what they produce with their labour. In the ideal world, therefore, money should be abolished.
A number of other premises and corollaries work together with this logic. One is the upper bound of demands, which I think can be fairly called obsolete. Another is that understanding demands is easy, which would be the case if demands are static. If that is so, allocation of resources would not be a trouble either. Marx was mainly concerned with the allocation between classes. He'd never talk about how to assess which specific factory should get materials, so we can only assume that he thought it's trivial. That takes away another use of money (as well as the market). Then, whether natural resources can be exhausted was never touched on either, probably suggesting that he didn't thought it'd happen, at least not if you don't waste it with capitalist overproduction. His refutation against Malthus was that the society ought be able to produce more with a better relations of production, rather than Malthus's way of forcefully limiting the number of people.
Finally, unlike today's greenish lefties who complain about capitalism's ruthless efficiency, Marx believed that capitalism was exceedingly inefficient. In particular, the contradiction between, on one hand capitalists want to reduce wages as much as possible; on the other hand they also want to sell as much as possible. That causes commercial crises, which destroys businesses, drives workers into unemployment, and depresses both demands and productivity. A communist economy will have none of that, therefore it will be able to provide for every person according to his needs.