If you're just working out of your home selling on your own Ecommerce store or through Ebay or other sites then you shouldn't need a business license to do so.
Although if you do have an online store that you own and operate yourself if you do get a business license and post the business license number on the website then it will make you more trusted and people will be more likely to buy from your online store.
Getting a business license can be pretty cheap and easy but check with your local zoning ordinances to be sure it's legal to operate a business out of your home.
Some places allow it and some places don't.
You may have to rent an office space to operate the business so be prepared for that.
But if you're just starting out with the online store then you should be able to get by selling and operating from your home without the license until profits and revenue allow you to rent an office space.