"External goods [i.e. commodities] have the character of means useful for an end. Hence man's good in them must consist in a certain measure of them; that is, a man must seek to have external riches only in a certain measure, insofar as they are necessary for him in his state of life. In any excess of the measure there will be sin; it is evil if he should wish to get or keep them beyond a right measure. This would be avarice, which is defined as 'the immoderate love of having.' ...-- St. Thomas Aquinas ST IIa IIae, q. 88, Art. 4, resp. 1.
Avarice can be immoderate in external goods in two ways. First, directly in the getting or keeping of these goods, by getting of keeping them more than he should. This is directly a sin against our neighbor, because external goods cannot be simultaneously possessed by many, and therefore, if one man has more than he ought, others have less than they ought.
Secondly, avarice can imply an immoderateness in the internal affection we have for riches, namely, by immoderately loving, desiring, or delighting in them.... Consequently, it is a sin against God."