For California, you need a seller's permit to sell things. Doesn't matter if you make them yourself or buy them and re-sell them. [Only thing that doesn't require a seller's permit is things that are not levied a sales tax, like food].
Call the State Board of Equalization to get the application - I think you can apply online too.
Depending on what you sell, you may also need a business license from your city. You'll need a DBA as well, unless your last name is a part of your business name.
Also, you should look into liability insurance.
I continue to find more and more requirements and fees for my business, especially since it's food! Aarrgh. I can see why so many people either stay under the table and risk everything, or just stop trying.