If you are on a desktop, the advanced search (as outlined by pj) is the way to go.
If you are using the ebay smartphone app (my version isn't the latest) you need only to type seller:name in the search field with 'name' being replaced by their user name and not their store name.
But you have to already know who you are looking for in order for that to work.
I think, from your question, you remember the name of the seller's store but not their user ID and wonder how to find them again? Use the 'follow' function and you'll never lose them again OR check your own feedback left or received to see who it was that you bought from before if you have had a transaction from them in the past.
It also works to google the store name and then follow the link to ebay. I do that all the time.
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