It is the middlemen who currently run this world.
Easiest way to make money.
1. Find a product that's valuable.
2. Buy the same thing at a lower price by purchasing in large quantities.
3. Sell individual units at a profit.
The giants Amazon, Alibaba, Costco, Walmart, ETC. all exploit this tried and true method of creating value.
I also sell products wholesale on Amazon and can tell you from experience this works. Anyone can do it.
The thing is, once people can make transactions directly from the warehouse or the seller without the need of a platform or place at the best price and with great shipping, security, trust etc. won't all forms of middlemen become obsolete?
Everything would be cheaper and more accessible.
If this is your definition of the middleman, then every businesses and stores you see around, are all middlemen! Every manufacturer sells their product to the wholeseller first. The wholeseller then sells it to the retailer from where we buy the products. As per your your definition of middleman, these wholeseller and retails businesses, are all middlemen. That's how exactly the a trade functions!
And due to this same reason, there is a healthy competition in the market. If everything goes to the hand of the manufacturer, this competition will turn ugly and we will become the victims of their cartel the way Banking system is showing off their power now!
If you think that decentralized marketplace will let you connect directly with the manufacturer, you're wrong! It's mainly a marketplace that allows peer to peer transactions without the need of having a third party to verify the activities for both buyers and sellers.
There are both pros and cons for the decentralized marketplace. A big challenge is the dispute resolution area. Lets assume, you purchased a mobile phone from a seller of a decentralized marketplace but the when the product arrived, you found manufacturing defects in it. Without any centralized authority, how will you get resolution or get the refund? That's a very practical challenge in today's scenario.
So more research and ideas are needed to overcome such obstacles. Decentralization is a very new structure to us and we will need more time before we can actually think of competing with the behemoths like Amazon, Walmart etc.