These are two different things:
1) the permission to join the show activity, which some craft shows address in the form of a prejudging (to be sure all vendors are of a similar artistic level). and then the
2) legal tax entity in the form of a wholesale license, reseller's license, or seller's permit. You can get one for a short term, like 30 days or as a permanent business.
You've already cleared any hurdle regarding #1 if you are welcomed to the show, and please remember that you should never have to pay to get information regarding #2 because this is normal state tax activity.
The object of the game is: The state wants to collect sales tax or otherwise be advised of business operation. The benefit is, that with a wholesalers or sellers' license, you can purchase your materials at a lower cost and tax free; it can often get you into areas of trade shows that the ordinary purchaser (the retail buyer) has no access. The down side is: welcome to business, you're now expected to keep records. But this stuff really isn't that hard.
The laws are different by state - so you will want your own state's websites, the examples I have (below) are California because that's were I live. Search on your state's name and Seller's Permit.
Seller's Permits are usually FREE, they can be had for short periods (like 30 days to cover just the one show) or ongoing. The records you keep are simple: what did you buy & what did you sell. You tally total goods sold and pay the sales tax to the state. It's a form and you write a check. So where did you get the money? Well, as a business - you charge your customers.
And how do you deal with that at the show? You could plan ahead, figuring what you were going to charge, what the tax would be and round up to an currency-even number so making change was not tedious. Whatever price you choose, plan ahead. If you make it easier for customers, it will encourage sales.
CA.gov "Who must obtain a Seller's Permit?"
http://www.boe.ca.gov/sutax/faqseller.htm
California Board of Equalization "Do You Need a California Seller's Permit?" http://www.boe.ca.gov/pdf/pub107.pdf
For California, where to get the forms (small business center) note the online tutorials on state tax: http://www.boe.ca.gov/
CA Seller's Application form: http://www.boe.ca.gov/pdf/boe400spa.pdf
Websites like this one, have a lot of general information well beyond the simple Seller's Permit that you need, but you can go through their checklists and learn something. Please remember, there's no reason to give them money to file for you - filing is FREE at your own state's office. http://www.businessnameusa.com/default.htm
http://www.sellerpermit.com/
Craft Business: Basics of Getting Started
http://www.smallbusinessbc.ca/pdf/crafts.pdf
Federal Tax info - & BTW don't get scared, lots of people have businesses on the side. If you're a single entity (not a partnership) all federal tax is on your own tax return just stated as additional income, when it's over a certain amount, federal tax laws kick in.
http://www.irs.gov/publications/p334/ch10.html
Barbra Brabec's Craft Business website - some good tips
http://www.barbarabrabec.com/crafts_marketing.htm