Signing up for Ebay is a simple process. There are a few Hoops to jump through initially but none of it is too difficult. You do not need a legal business name to get started on Ebay. You can start selling today after just going through the basic registration steps. The registration simply means your name and address and a payment method for fees.
Ebay lets you post your first 50 listings every month free of charge and only charges fees on the back end when something sells. Ebay makes most of their money from the final value fees which are 10% of the sale. that gives you a lot of freedom and ability to grow a business on their platform for very little. After the first 50 listings which are free they start charging a small $0.35 insertion fee for additional listings. Even this can be lowered over time when you sign up for a store but there is no need for an eBay store in the beginning.
Here is a video that walks you through the registration process for Ebay.