Sale of anything, constitutes commerce. Either side can be taxed, (e.g. Duty paid by the seller, excise paid by the purchaser of (goods, property, labor or service).
Charles C. Stewart Machine Co. v. Davis (1937), 301 U.S. 548:
House Congressional Record, March 27, 1943, pg. 2580:
Even both sides of the transaction can be taxed, as in the FICA.
HELVERING v. DAVIS, 301 U.S. 619 (1937)
The primary factor to distinguish is whether or not property is taxed merely upon the fact of ownership thereof or not. If an exchange or event is involved in transfer of ownership from one person to another, then the tax is deemed indirect as a consequence of being in the stream of commerce.
The Records of the Federal Convention of 1787
(Farrand's Records)
James Mchenry before the Maryland House of Delegates.
Maryland Novr. 29th 1787--
Appendix A, CXLVIa, page 149, S9.
"Convention have also provided against any direct or Capitation Tax but according to an equal proportion among the respective States: This was thought a necessary precaution though it was the idea of every one that government would seldom have recourse to direct Taxation, and that the objects of Commerce would be more than Sufficient to answer the common exigencies of State and should further supplies be necessary, the power of Congress would not be exercised while the respective States would raise those supplies in any other manner more suitable to their own inclinations --"
A LAW DICTIONARY
by John Bouvier, Revised Sixth Edition, 1856:
"COMMERCE, trade, contracts.
The exchange of commodities for commodities; considered in a legal point of view, it consists in the various agreements which have for their object to facilitate the exchange of the products of the earth or industry of man, with an intent to realize a profit. Pard. Dr. Coin. n. 1. In a narrower sense, commerce signifies any reciprocal agreements between two persons, by which one delivers to the other a thing, which the latter accepts, and for which he pays a consideration; if the consideration be money, it is called a sale; if any other thing than money, it is called exchange or barter. Domat, Dr. Pub. liv. 1, tit. 7, s. 1, n. "
As opposed to static ownership of property by a single person or entity to which direct taxes apply.
I pay both sides of my fica (self-employed) but then so does everybody else. The employer really doesn't pay any of it.