When is my nexus start date?

To our understanding, the states consider that you have nexus in their state as soon as you engage in nexus creating activities (including storing inventory) in their state. 

• If you sell via FBA, you can determine the date Amazon first stored your inventory in the state by viewing your Amazon Inventory Event Detail reports.

If you're in the process of registering for a sales tax license and you need to determine your nexus start date, we would recommend making this decision alongside your CPA or tax consultant. 

• You may also want to speak to the states' DOR directly to ask for more guidance.

What should I do about taxes I didn't collect before I registered?

Though we're not able to specifically advise you regarding what you should pay the state in sales taxes for sales you made prior to registering, we do have a couple of blog posts that outline some choices you can consider in this situation:
1)  Voluntary Disclosure Agreements
2)  State Tax Amnesty Programs 
3)  When to register for a sales tax license

We can also refer you to a  sales tax professional who would be able to specifically help you assess your next steps!