Do I need an LLC?
If you are making that much from your online sales, it's certainly a good idea to have an LLC for a few reasons. One of the biggest reasons being the protection of yourself. Once you start showing profits - and subsequently assets - as a sole-proprietor, it puts you on the radar where someone may be tempted to see what they could get out of you with a simple petty lawsuit.
Forming an LLC would also grant you a new way to advertise and grow your customer base, generating more sales under a credible name. The LLC would be it's own entity, complete with it's own bank account and credit line. This would also aid in expansion and growth of your business as well.
Additionally, I am assuming you conduct your business on Ebay from home? If that is the case, having your home address set as your principle LLC address should entitle you to write some of your rent and utilities off from your taxes. It is always a good idea to consult with your accountant as he or she would be able to help outline a developmental plan that would be the most beneficial to you and your business.
I hope this answer was helpful.