Do you need a resell certificate for Ebay sell > $20k/year?

I start buy and sell on ebay regularly this year. I check the state law it says you need to obtain a seller certificate even to collect state tax if the buyer is from the same state. Then do you need to form a LLC? I am selling part time and hate the hassle dealing with paperwork once you have a LLC even if it makes little profit. I will report my income on tax return  but will be trouble if I do not have a license or business name? Thanks!