How is Surplus Value Possible?

Not from buying cheap and selling dear: that would be a zero sum game if everyone did it.

Marx uses abstract labor, necessary labor, and surplus labor to explain:  necessary labor is the labor required to reproduce the society at a certain material and cultural level.  Surplus labor is the labor performed above and beyond this amount.

After a certain level of productivity has been reached, a person or group is able, by the consumption of certain use-values, to labor for a time longer than that necessary for the production of the use values requisite for reproducing the labor power thus realized as labor.

For example, a working day might be divided into two parts: a necessary part and a surplus part. In the necessary part, a person produces enough to reproduce their labor. In the surplus part, they produce the surplus. The capitalist buys labor with wages that are equal to the cost of its reproduction...the necessary labor, and appropriates the surplus labor gratis.