I've heard this specific dream pitched before and while I think it's a great idea it still doesn't solve the issues around shipping specifically. One of the things Amazon does really really well is the logistics of shipping the product to the customers and while a bunch of people coming together to buy goods straight from the producer would cut out the profit taken by the middleman it wouldn't help with getting the items to your door. Individual manufacturers simply can't negotiate the contracts that Amazon has with UPS and individual manufacturers don't have the the warehouses and the massive infrastructure Amazon has. If a producer made 10,000 widgets and sent them out to 10,000 people that is a lot more difficult than simply sending a pallet to Amazon and having them sell the items and send them to the customers. They aren't just some website that lets you buy things, there is an enormous infrastructure that supports the commerce.
This is something you can get a better sense for when you back Kickstarter projects and see just how much time is spent around packaging and shipping. It is a huge task that a lot of inexperienced companies get totally swamped by.
Edit: I would also like to add that if we bought goods straight from the manufacturer then we would have to deal with greater ship times as well. If you're ordering something from a factory across the US it's going to take a week to get to you. But if Amazon or another marketplace has already purchased the item and stores it a day away from you or in the same city (knowing that there is a chance someone like you is going to order one soon) then you can get the item much faster. That convenience is something that really only a logistics and warehousing middleman can do effectively and people pay for that convenience.
Edit 2: I don't work for or am affiliated with Amazon.com - I just have seen into the belly of the beast at a few online retailers and have good sense of what goes on in there.
UPS is going out of business, too. UPS owns warehouses, trucks, and planes, none of which are very defensible advantages. Like the companies I listed, they're a network effect company. Those companies are going away because bigger networks can be formed on the global computer by anyone who wants to participate—warehouse owners, truck drivers, bike messengers, etc.
UPS gives Amazon a good deal because they push so much volume, and it's way easier to sell to one company than it is to sell to millions of them. But the global computer makes it feasible to directly connect every producer with every consumer—it won't be hard to sell to millions of them. If a shipper needs guaranteed future business, customers can band together and put a contract for lower prices on the global computer that enforces a payment or reputation penalty if a minimum spending limit isn't reached. Decentralization gives everyone the economies of scale that once required the backing of a megacorporation.
Most of the advantages of scale are reducible to middleman-free interactions on the global computer.
This is something you can get a better sense for when you back Kickstarter projects and see just how much time is spent around packaging and shipping. It is a huge task that a lot of inexperienced companies get totally swamped by.
Edit: I would also like to add that if we bought goods straight from the manufacturer then we would have to deal with greater ship times as well. If you're ordering something from a factory across the US it's going to take a week to get to you. But if Amazon or another marketplace has already purchased the item and stores it a day away from you or in the same city (knowing that there is a chance someone like you is going to order one soon) then you can get the item much faster. That convenience is something that really only a logistics and warehousing middleman can do effectively and people pay for that convenience.
Edit 2: I don't work for or am affiliated with Amazon.com - I just have seen into the belly of the beast at a few online retailers and have good sense of what goes on in there.