How we cite our quotes: (Chapter.Section.Paragraph) or (Chapter.Paragraph)
Quote #4
The first distinction between money as money and money as capital is nothing more than a difference in their form of circulation. The direct form of the circulation of commodities is C-M-C, the transformation of commodities into money and the re-conversion of money into commodities: selling in order to buy. But alongside this form we find another form, which is quite distinct from the first: M-C-M, the transformation of money into commodities, and the re-conversion of commodities into money: buying in order to sell. Money which describes the latter course in its movement is transformed into capital, becomes capital, and, from the point of view of its function, already is capital. (4.4)
Selling in order to buy (C-M-C) and buying in order to sell (M-C-M) are the two fundamental processes of market exchange. It's what we do with wealth, our commodities. Capital is the latter process. That means Marx doesn't picture capital as an unchanging thing, but instead as something in motion, a type of circulation of commodities.
Quote #5
The process M-C-M does not therefore owe its content to any qualitative difference between its extremes, for they are both money, but solely to quantitative changes. More money is finally withdrawn from circulation than was thrown into it at the beginning. The cotton originally bought for £100 is for example re-sold at £100+£10, i.e. £110. The complete form of this process is therefore M-C-M', where M' = M + ΔM, i.e. the original sum advanced plus an increment. This increment or excess over the original value I call 'surplus-value'. (4.15)
There's no reason to buy in order to sell (M-C-M) unless you think you're going to come out of the process with more money. The more money you end up with if you successfully buy in order to sell at a higher price is called surplus-value by Marx. Think of surplus as meaning extra, like the extra items that make up the stock at a surplus store.
Quote #6
The simple circulation of commodities—selling in order to buy—is a means to a final goal which lies outside circulation, namely the appropriation of use-values, the satisfaction of needs. As against this, the circulation of money as capital is an end in itself, for the valorization of value takes place only within this constantly renewed movement. The movement of capital is therefore limitless. (4.17)
Selling in order to buy means you wind up with a use-value, which you then consume, such that the commodity leaves the market exchange system (the sphere of circulation). But buying in order to sell keeps the commodities in circulation; the process keeps going and going. That's why capital (the process of buying in order to sell) can keep seeking surplus-value forever, at least as long as the market and the commodities continue to exist.