Our Gemara brings a baraita that emphasizes the severity of shemitta, in that even forbidden business dealings with shemitta fruit, which is referred to as avakah shel shevi’it – “the dust of the Sabbatical Year laws” – leads to severe punishment. Calling the laws forbidding business dealings with shemitta fruit avakah shel shevi’it indicates that these are not the main rules of the Sabbatical Year, either because the focus of the prohibitions of the Sabbatical Year is the agricultural work itself (Rashi and Tosafot), or because business dealings are only a small part of the larger prohibition against storing the kedushat shevi’it fruit for use in the year after shemitta (Arukh).
According to Rabbi Yosei bar Hanina in the baraita, someone who does business with Sabbatical Year fruits will find himself impoverished and be forced to sell his movable property. Should he not realize the severity of his actions, the punishment will continue as long as he does not repent. He will end up selling his agricultural lands, then his own house and real estate; he will be forced to take loans, and when he cannot pay them he will be forced to sell himself as a slave to a fellow Jew – or even to a non-Jew.
Rabbi Yosei bar Hanina derives this from his reading of the laws of shemitta as they appear in the Torah, and the continuation of the laws that are discussed there. In Parashat Behar (Vayikra 25:1-13) we learn the rules of shemitta and yovel. This is followed by the laws of buying and selling movable objects (pesuk 14), selling land (beginning with pesuk 15), selling homes (beginning with pesuk 29), borrowing money (from pesuk 35), being sold as a slave to a Jew (from pesuk 39) and finally being sold as a slave to a non-Jew (47 and onwards).