Looks like eBay does not allow buyers to filter sellers.
Only the other way round...
Here's my solution, next to the search button above click Advanced
Then I type my keywords, choose a category, and down below I add the sellers IDs and separate them with just a comma.
I click search.
Then I click the save search so I can reuse that search criteria.
But I find it easier to save it in my bookmarks and whenever I've another seller ID that I never want to waste my time on again, I edit my bookmark, find the first Seller ID and type %2C followed by the new seller ID I want to filter.
Neither method is very good, but they work for now...