Contrary to popular belief, it is not the consumer's responsibility to pay the sales tax. The business is required to report sales to the state every month to keep their business license, and for all in-state sales that business is required to pay sales tax. For sales that originated from and shipped to a different state, the business has to report the sale (all sales), but does not have to pay sales tax. If a business sells an item in-state, they must report the sale and pay all related taxes to the state on a monthly basis. Since businesses don't like people cutting into their proift margins, they pass this to the consumer and make them pay the sales tax. Over time, the general public just pays it without questioning it and businesses make the consumer think that they are responsible for paying them. Consumers are not required to pay sales tax, the BUSINESS is. Paying taxes on in-state sales is the cost of doing business and should not be passed on to the consumer. J and K Stuff doesn't charge any tax on in-state sales as it's our cost of doing business. Whatever taxes we have to pay comes out of our profit.
Use tax is a different story.
Regarding your ebay sales and charging taxes...
1.) If you have a legitimate business license, you are required to report in-state sales monthly or quarterly. If you aren't a business, you only have to report profit from sales on your income tax.
2.) If you paid taxes on an item and re-sell it in-state, you don't have to pay sales tax to the state, since you already paid taxes when you purchased it.
As a legitimate Limited Liability Corporation (LLC) business here in AZ with a Federal Employer Identification Number (EIN), we found all of this out when "setting up shop" and maintaining the business license. BTW, a valid business license is required to purchase at true wholesale prices and manufacturers and distributors require one to sell to you, just FYI.
I am not a tax professional by any stretch of the imagination, this is stuff we've figured out on our own. It's written right there in the AZ tax code and requirements so it doesn't really take a professional to figure it out.