As I understand it California for example would claim you have a nexus once any stock at all has been moved into an Amazon warehouse in their state. In that cas

As I understand it California for example would claim you have a nexus once any stock at all has been moved into an Amazon warehouse in their state. In that case all you would need to see is that Amazon has moved some of your stock into CA and then you would have a nexus and owe sales tax on any sales to customers in CA that your business does, regardless of where that stock comes from. To clear yourself of this nexus requirement for CA you would need to have had no stock stored there for a 6 month period. Obviously whether Amazon internally transporting one unit of your stock to CA for a few days constitutes a "substantial presence" is an untested legal question.