Nice link Bianca :).
Within the U.S., you do not have to charge your customers tax unless they are within your own state. If they are within your own state, you do not have to charge them sales tax if you both have tax id #'s. Otherwise, they are supposed to be charged sales tax.
As far as claiming taxes goes, you are supposed to claim online sales profits just as all other income. I doubt that many ebay sellers do but if you make too much they may get ya :shock: .
If you plan to really get serious I would get a state/city business license, a federal tax ID, and claim your income accurately. If you're selling here and there, I suppose it's up to you.
BTW tax regulations vary from state to state. The info I just provided relates to states which I have lived in and many others that I am familiar with. Contact your local gov. to get the details.