The theory of buying and selling becomes therefore much more complex when we take account of the dependence of marginal utility on amount in the case of money as well as of the commodity itself. The practical importance of this consideration is not very great. But a contrast is drawn in Appendix F between barter and dealings in which one side of each exchange is in the form of general purchasing power. In barter a person's stock of either commodity exchanged needs to be adjusted closely to his individual wants. If his stock is too large he may have no good use for it. If his stock is too small he may have some difficulty in finding any one who can conveniently give him what he wants and is also in need of the particular things of which he himself has a superfluity. But any one who has a stock of general purchasing power, can obtain any thing he wants as soon as he meets with any one who has a superfluity of that thing: he needs not to hunt about till he comes across "the double coincidence" of a person who can spare what he wants, and also wants what he can spare. Consequently every one, and especially a professional dealer, can afford to keep command over a large stock of money; and can therefore make considerable purchases without depleting his stock of money or greatly altering its marginal value.