Marx is talking about using precious metals as money.
Buying: money form -> commodity form
Selling: commodity form -> money form
But if you are mining "money", at the point it enters circulation it is not yet in money form, since you produce it as a commodity to be exchanged for concrete use-values. For you it is commodity form -> commodity form. But for the person you exchange it with, it will be already in money form, therefore for them it is commodity form -> money form, i.e. selling.
If you have only buying and selling, the amount of money in circulation couldn't grow. This is how it grows anyway.