Commercial activity does not require trade. Otherwise, professional services such as accounting or law would not be commercial activities since no "trade" actually occurs. Commercial activity simply requires an intent to achieve a net economic benefit position (i.e., profit) through an activity.
Moreover, trade that does not cross state lines can be interstate. For example--you make good Widget that you sell nationwide. I decide in State X that I want to make and sell Widget as well. (Patent, etc. are not a concern in this hypothetical). My sale in State X of widget necessarily affects your interstate sale of the same widget: every widget that I sell in State X is one less that you might sell, which means one more widget is available in other states, altering the normal supply/demand considerations. (I'm not just pulling this out of my ass, this is the actual logic set forth in the SCOTUS cases.)
Moreover, trade that does not cross state lines can be interstate. For example--you make good Widget that you sell nationwide. I decide in State X that I want to make and sell Widget as well. (Patent, etc. are not a concern in this hypothetical). My sale in State X of widget necessarily affects your interstate sale of the same widget: every widget that I sell in State X is one less that you might sell, which means one more widget is available in other states, altering the normal supply/demand considerations. (I'm not just pulling this out of my ass, this is the actual logic set forth in the SCOTUS cases.)