Actually, Liz, I think a Hub writer need not have any eBay accounts to use the eBay capsules. Let's look at both scenarios.
If you don't have an eBay account, click on the eBay capsule, and you will have an opportunity to use key words that might help pull up items that are pertinent to your subject matter.
If you do have an eBay account but are only a buyer, not a seller, the previous paragraph applies.
In addition, if you know an eBay user's ID and his or her products are pertinent to your Hub's subject matter, instead of using key words to search for those items, you can try putting the eBay user's ID in the appropriate window.
Next scenario, If you have an eBay account and are a seller, all of the above alternatives apply.
Bonus: If you have an eBay account and are a seller, AND THE ITEMS YOU SELL ARE PERTINENT TO YOUR HUB TOPIC, you can type in your user ID and thus display your own products. But they must be pertinent to the subject matter. Very important!
Good luck to you!