Commentary for Beitzah 73:7

On account of buying and selling - that buying and selling is similar to take out from his domain and transfer the object to the holy domain [the Temple]. This action is forbidden by Torah which states "[honor the Sabbath by not] seeking out your personal desires, nor discussing [forbidden] matters." (Isaiah 58:13). Or alternatively, buying and selling will cause someone to write a bill of sale. And if you would say...