Middle men are usually characterized as sellers that don't produce their own product, that sell widely available products. Instead they buy from factories on demand, acting as a middle man between you and the factory. Sometimes they are also called resellers.
This terms still has a very negative connotation in our community, although it is a very common practice in almost every industry. "Cut out the middle man" is (falsely) seen as a solution for everything.
I think you have to take a more unemotional look at the topic. It is hard to imagine that a single person would be able to decide what products the market demands, handle the production, marketing, store keeping, sale and logistics.
Middle men cover the marketing, store keeping, sales and often times the logistics. Which means they play a big part in the value chain. Of course they will add a profit margin to their price. Cutting them out is easier said then done: even if you find the same product it's most likely from another middle man, whether you want to believe it or not, because the factories usually don't sell directly to consumers.
Which doesn't mean that there aren't better alternatives to some of the most popular middle men. Especially for common items that are easy to compare you can find cheaper offers on taobao or weidian.
So to sum it up, almost any seller you're dealing with is a middle man to some extend. For very specific items that only few sellers carry it will be hard to find a worthwhile alternative. For widespread items (most sneakers) it's worth looking for alternatives. But don't forget