[ghaff]

Specifically, by default, mail order stores are not required to collect sales tax unless the have a physical presence in the state (retail store, sales office, distribution center, etc.) Quill Corp. v. North Dakota from 1992 is the major SCOTUS case law.

In theory--at least in most states with sales tax--residents are supposed to pay a use tax on out-of-state purchases bought to be used in their state of residence.

A number of states have put pressure on online retailers like Amazon to collect. One of their levers has been to argue that Amazon Associates constitute a physical presence. And Amazon has agreed to collect sales tax (although not for affiliate sales) in several states. Massachusetts for one.