eBay does not give out names or any information based on names or numbers, only by username.
If you know the seller's username, you can search for the seller's items for sale on the Advanced Search page. Using a desktop browser you can find a link next to the search button on most eBay pages.
If you viewed one of the seller's items recently, check your Recently Viewed page to see if you can spot it:
https://www.ebay.com/mye/myebay/rvi
If you find an item you are interested in, add it to your Watch List to keep track of it. If you find a seller you are interested in, save the seller to your Saved Seller list.
