Individual items can be added to a Watch List.
Sellers can be added to your Saved Seller list.
To find a seller's items for sale by username, use a desktop browser to go to the Advanced Search page to search by seller:
https://www.ebay.com/sch/ebayadvsearch?_sofindtype=0&_byseller=1#byseller
From there you should be able to watch an item or save the seller, if the seller has any current or recent listings.
