Well, if you want to buy something from eBay then ask an adult with a creditcard to assist you. I'm sure there are other ways to find a twintub, look at the ads in your local newspaper and have a look at thriftstores.
One doesn't always need a credit card to buy from e-bay, the payment terms are dictated by the seller, not E-Bay. A lot of sellers DON'T rtake credit cards and require, Money Orders, Certified Checks, or personal checks.