I think due to South Dakota v. Wayfair, states now have more power to define what constitutes "nexus"... so there are all these special rules and thresholds now

I think due to South Dakota v. Wayfair, states now have more power to define what constitutes "nexus"... so there are all these special rules and thresholds now to keep track of (different for every state) that define whether you have economic nexus and need to collect sales tax there. If you hit the threshold and/or other rules apply, you're obligated to collect sales tax... but of course you can't do this until you have secured a license in that state. This was my read on the situation, but if I'm missing something definitely let me know.