Filtering by seller doesn't show up consistently-- I've yet to identify what exactly the pattern is, but, once you've drilled down far enough into their search

Filtering by seller doesn't show up consistently-- I've yet to identify what exactly the pattern is, but, once you've drilled down far enough into their search results, it usually shows up for me. It very rarely shows up for me after a general search (from the search bar on the Amazon homepage), though-- for example, if I search for "T-shirt" from the homepage, I can't filter by seller, but, if I drill down to the "Men's Shirts" category in the left-hand bar on that search results page, I can filter those results by seller (and select "Amazon.com" to see things they claim they're selling themselves).