So I've spoken with a tax attorney (my business law professor) who gave me kind of a final word on this subject. Basically, states cannot charge sales tax on goods that are purchased through an internet vendor not located with in the state. However, states can charge a use tax on products purchased outside of the state. The only way they can obtain this money though is through an audit, and most likely they will not audit an individual as it is not very cost effective. Ironically, her business got audited by the state tax authorities for internet purchases and was forced to pay back taxes, fees and interest. In addition, in my state, Maryland, the former treasurer had gone after furniture manufacturers in North Carolina for shipping to Maryland without charging for taxes. Basically, the state can force you to pay use taxes on products purchased online, but it probably will not because they are looking for other ways to more easily recover sales tax revenue lost to online retailers such as a flat internet sales tax, etc. If you are really paranoid, you can report all of your internet purchases and pay the government the requisite taxes, but otherwise you don't really have to worry.
In answer to the question on requesting information from vendors, it's not really a question of probable cause, but of jurisdiction because the company already does not recover taxes from its customers. If it did, you could report it and if there was a positive difference between the sales tax charged and the use tax in your home state, you will receive a tax credit. With respect to jurisdiction, it could easily get the information from in-state vendors and from out of state vendors, it could prove long-arm jurisdiction if there was a significant number of customers from within the state.
About the forms that individuals were receiving, in all likelihood they were just a fishing expedition. However, my professor said that if the government wanted, they could probably compel you to fill out the form.
The government just wants to recover its lost taxes and constitutionally they do have the right to tax us, so we will have to pay, or pay the consequences, whatever they may be. If you choose to wait until the government compels you, you may have to pay more, but hedging your bets is up to you.
This was pretty long, but if you have any other specific questions let me know and I'll try to answer them.