If you're already selling and making small profits off of eBay sales, your social security number is a suitable enough tax ID number. But since eBay has its own policies--and since the IRS's policies change as legislation changes--you'll want to familiarize yourself with eBay and the IRS by going to their websites and reading the fine print.
Also speak to your accountant--as you may have sales tax issues (for which you need a special ID number for that) in connection with the collection of sales tax on the items you're selling.
Nina L. Kaufman, Esq. is an award-winning New York City attorney, edutainer and author. Under her Ask The Business Lawyer brand, she reaches thousands of entrepreneurs and small business owners with her legal services, professional speaking, information products, and LexAppeal weekly ezine. She also writes the Making It Legal blog.