Capitalism does not urge one to define one's purchases... capitalism is simply man's natural state of
freedom, before he limits himself through
governmental or
social mechanisms and as a resault, it neither says that one should or should not define one's self through one's purchases, it only indicates a state in which man is free to either define himself through his purchases or not, by his choice.
In capitalism, it is true, however, that man is defined by his purchases (or the lack thereof) as, by definition, in capitalism a man may choose what he buys and what he doesn't by, and any choice that a man makes, is a step towards defining the man who makes them.