"The Supreme Court issued several opinions supporting that use of the Commerce Clause. Heart of Atlanta Motel v. United States, 379 U.S. 241 (1964), ruled that Congress could regulate a business that served mostly interstate travelers."

Heart of Atlanta Motel made the argument that people who wanted to rent a room were neither goods nor services which crossed state lines, and argued they had a constitutional right of association so should be free to racially discriminate.

The Interstate Commerce Clause even applies to crops you grow for yourself, which aren't on the market. From https://en.wikipedia.org/wiki/Wickard_v._Filburn

"An Ohio farmer, Roscoe Filburn, was growing wheat to feed animals on his own farm. The U.S. government had established limits on wheat production, based on the acreage owned by a farmer, to stabilize wheat prices and supplies. Filburn grew more than was permitted and so was ordered to pay a penalty. In response, he said that because his wheat was not sold, it could not be regulated as commerce, let alone "interstate" commerce (described in the Constitution as "Commerce ... among the several states"). The Supreme Court disagreed ... The Court decided that Filburn's wheat-growing activities reduced the amount of wheat he would buy for animal feed on the open market, which is traded nationally, is thus interstate, and is therefore within the scope of the Commerce Clause. Although Filburn's relatively small amount of production of more wheat than he was allotted would not affect interstate commerce itself, the cumulative actions of thousands of other farmers like Filburn would become substantial. Therefore, the Court decided that the federal government could regulate Filburn's production."

This interpretation also applies to medical marijuana, quoting again https://en.wikipedia.org/wiki/Commerce_Clause

"In a 2005 medical marijuana case, Gonzales v. Raich, the U.S. Supreme Court rejected the argument that the ban on growing medical marijuana for personal use exceeded the powers of Congress under the Commerce Clause. Even if no goods were sold or transported across state lines, the Court found that there could be an indirect effect on interstate commerce and relied heavily on a New Deal case, Wickard v. Filburn, which held that the government may regulate personal cultivation and consumption of crops because the aggregate effect of individual consumption could have an indirect effect on interstate commerce."