Here's the deal: the government through force of law has prohibited other free-market venues; when it comes to money paid to the government in order to obtain something, no matter whether it's a building fee or a driver's license or a boat registration, it's still a government monopoly and the prices it charges are a form of taxation.
In this particular example of highway signage, the government has skewed the free market by prohibiting private billboards, big towering pole signs, etc. The McDonalds and Arby's and mom-and-pop restaurants on the sides of interstate highways used to make their own signs and they worked great (until nanny staters decided they were just too ugly!!! and used the law to step in and screw up the private marketplace); those businesses would probably NEVER opt for space on highway sign over a private billboard of their own -- but the government has outlawed all options other than not advertising at all. Granted, that is a choice, but a losing choice -- so these businesses are in affect forced to engage in what you call a buy/sell transaction, but which is really a form taxation because the money is charged by and paid to a government entity.
***— it’s an easy mistake to make, and I sympathize.***
%%%%%
LOL!
We will have to agree to disagree. You have your definition of ‘tax’ and I have mine.