The state that has jurisdiction over an interstate sale will be the state where delivery occurs.
This is usually where physical control or the right of control transfers from the seller to the buyer. Since, in an interstate sale, the only thing that can be taxed is the buyer's use of the TPP, and since use involves control over the TPP, then the delivery point determines, for all practical purposes, the state that gets to impose the use tax.
In other words, where is the delivery point?
Another way to look at it is where the seller completes their duties. This is particularly helpful when you're dealing with a situation involving a long installation.
A few (and I do mean a few) states use FOB points to determine the state that has jurisdiction. That can be overcome by the buyer negotiating FOB destination.
Use of this Golden Rule will help you resolve complex transactions to the specific state whose rules need to be followed.
There's a corollary to this rule
There's a little mini-course on this rule.
The Sales Tax Guy