You can hit Amazon with niche categories. For example they have shoes and clothing, but there is no "Amazon for nurses scrubs or school uniforms" or other niche categories. They also do not serve many countries (e.g. Philippines). A startup that served these needs would not have Amazon as a competitor at all.
Out of curiosity, I just searched Amazon for "school uniforms" and there's a lot of stuff there. (Makes sense, since there are millions of kids in private and parochial schools - it's a big market.)
Even if you manage to find a retail category that Amazon is not currently involved in, you might wake up tomorrow to find that they've suddenly become your competitor.
You have found a category that Anazon serves, but does not serve well. I buy nearly everything on Amazon but I still have to buy her uniforms in person (each school has a specific color scheme, plaid pattern, collar style, etc.). My wife still buys her nurse scrubs from a guy who comes around the hospital and only takes cash. These are things you just can't get easily on Amazon, even though it looks like they have that category covered.
Also, you could compete with Amazon on any category just by having a bigger selection of same day delivery in some niche.
