Keep a record of all that you read, all the people from whom you glean advice and of the process that you go through to sell something on eBay.
That is the basis for a Social Social Studies Project. The question: How difficult is the challenge of a teen selling on eBay?
It will probably entail getting a bank account (consider opening a Credit Union account, please) and a Paypal account.
Regardless whether you sell anything it will make a great story and project.