Note: We are not tax or legal advisors. Everyone situation is unique and you should consult with an expert.
In general, an LLC is not required for this business but some type of corporate entity is strongly suggested.
You do not need an LLC to open your Amazon Seller Account but you will need some sort of business registration to get your resellers permit.
A less expensive alternative to an LLC is a sole proprietorship or DBA that you can get in most states for free or as little as a $100.