Summarize:

If you're not already signed into your account, click Sign in near the top-left corner of the page to do so now. Use this method if you have an eBay item number and want to search for the person who sold it. It's near the top-right corner of the page. This opens the Advanced Search form, which allows you to search items from certain eBay sellers. It's in the left sidebar, near the top.  This displays a link to the item's listing. You'll find the seller's username under "Seller information" on the right side of the page.  From here, you can contact the seller by clicking Contact seller under "Seller information" on the right side of the page. To view the seller's profile, click their username. It's under "Seller information" on the right side of the listing. This adds the seller's active listings to your eBay homepage/feed.  You can also add a seller to your favorites by clicking  ♡ Save at the top of their profile. To view and manage your saved sellers, click My eBay at the top-right corner of any page, then click Saved sellers in the left panel.
Go to https://www.ebay.com in a web browser. Click Advanced next to the Search button. Click By item number. Type the item number into the box. Click Search. Click the title of the listing. Click ♡ Save this Seller if you'd like to add the seller to your favorites.