Ahhh...now you guys will know what I do for money!! I am a tax accountant by trade...artist for love. Anyway, I am experienced in sales tax. There is a site that is very good... it is www.aicpa.org
In it, you will find a place called CPA links, which will link you to all the state laws, forms, regulations that you could ever hope to know.
My advice would be this...when you are going out of state to exhibit and sell, check with the organizers of the show. They will be the fastest way of letting you know if you have to get a seller's permit for that particular state...or also if you need to file a sales tax return. Also, the site that I mentioned above, has print out forms for most states that you can use to print out those pesty little sales/use tax forms.
Internet selling is completely different. If you sell on the internet---individually and not through an organization...like artistnation...sales tax is handled (currently) like a catalog company. If you do not have "physical pressence" in the state but only advertise via common carrier (mail, internet) you usually do not have to pay sales tax in that state. But make sure to check your own laws and regulations before assuming anything about taxes!!! These laws change very fast...and congress has been working day and night for the past 5 years to determine, if any, taxation for the internet.
To the taxation officials--ignorance is not bliss. Just because you do not know of a law or tax rule in a state, does not exempt you from paying interest and penalties for not filing!!!
Now that I scared the crap right out of everyone....please...just use common sense. Try and ask questions of the officials handling the shows....consult the site I mentioned above. If there is still ambiguity in the answer, don't be afraid to call the sales/use tax division in the individual state department of revenue. They usually have their numbers listed on their websites.
Hope I didn't give anyone any gray hairs....