In practice, it's not. When used in the sense of establishing federal jurisdiction, "interstate commerce" means "all commerce Congress has the power to regulate," which is virtually synonymous with "all commerce in the USA." Like most areas of federal law (tax, wage & health, drugs, guns, etc.) trademark law fully applies to purely intra-state sales. Technically, there should be some intrastate element lurking somewhere (e.g., some piece of equipment, office supplies, fertilizer, etc. crossed a state line at some point), but even if not, an intra-state sale "affects" interstate commerce, which the Supreme Court has found sufficient for federal law to apply.