It was South Dakota v. Wayfair, Inc. I was a sysadmin so only tangentially involved, but the programmers were having a hard time because the situation was basically unknowable, That is, there was no primary source of information, I think they ended up buying a service that specialized in keeping track of what address charged what taxes.

https://en.wikipedia.org/wiki/South_Dakota_v._Wayfair%2C_Inc.

The basic idea is that some localities charge a use tax, which is a sales tax applied to items bought outside their jurisdiction. The citizens if the jurisdiction are required to self report and pay this tax. The problem(or beneficent depending of what side of the transaction you were on) is that this is very nearly unenforceable, just too large an attack surface. The standard way to enforce tax payment is to move it a rung up the ladder. The store collects collects the sales tax before you get the item, your employer collects the income tax before you see the money etc. now there are exponentially fewer collection points. the collection points are hidden, out of the voter base and each point has far more to loose if they rebel keeping them in line.