מדרש על בבא מציעא 175:6

And whence do we derive (the same for) what he sells? From "which goes forth from the field." They said: Why were the shops of Bnei Chanan laid waste two years before (the destruction of) Eretz Yisrael? Because they exempted their fruits from the tithe, expounding "Tithe shall you tithe … and you shall eat" — and not what you sell; "the produce of your seed" — and not (that of) the buyer.