Regarding this question, to buy an iron helmet on eBay, you can follow the steps below:

1. Open eBay's official website (www.ebay.com).

2. Type "iron helmet" in the search bar and click the search button.

3. On the search results page, you can use the filter options to narrow your search, such as price range, item status, seller location, etc.

4. Browse the search results and click on the helmet of interest to get more information, including product descriptions, seller reviews, etc.

5. If you decide to buy a helmet, you can click the "Buy Now" or "Add to Cart" button.

6. If you click "Buy Now", you will be redirected to the checkout page, where you will need to provide information such as the mailing address and payment method to complete the transaction.

7. If you click "Add to Cart", you can continue to browse other items or go to the shopping cart to checkout.

8. On the checkout page, you need to provide the required information and select a payment method to complete the purchase.

Please note that before purchasing, it is recommended to carefully read the seller's product description, return policy, seller reviews and other information to ensure that you purchase satisfactory products.