Let's just reduce this post to "Bitcoin is backed by nothing" type of argument and its value is determined by the people who uses it.
Let's suppose that you bought all the fiat currencies in the world and you decide not to sell them. How can you benefit from them? Well you can wait a couple of months and then visit the banks that issued them to claim an enormous wealth in the form of land, buildings, machines, equipment and other property that the banks seized from the borrowers.
The banks can simply print as many fiat as they can and they will never run out of it. They can also declare that those fiats you bought to be out of circulation and just devalue them.
Finally, let's suppose that you bought all the bitcoins in the world and you decide not to sell them. How can you benefit from them? To put it another way. After said purchase, all you have is a number in a computer memory - 18.232.575 (bitcoins currently in circulation). Let's suppose that I open my Excel spreadsheet and put the same number into it -> 18.232.575. Now we both have equal records in a computer memory.
...but no one else can prove ownership other than the owner of the wallet keys/seed containing those bitcoins by signing a message. You can have as many people with spreadsheets and the same number but can they prove that they really have it? NO.
Can you buy something and send bitcoin as payment just by using a spreadsheet? NO.
Shure, we can reduce this post to "Bitcoin is backed by nothing" type of argument and its value is determined by the people who uses it. But that's true for all ponzi-like schems - they are all backed by nothing and - their value is determined by the people who uses them.
Regarding buying and selling with bitcoin. Well, the whole point of this topic is the question how can you benefit from bitcoins without selling them. Selling them is how this scheme operates in the first place. All ponzi-like schems operate by selling their membership stakes to the new investors. So, the question is how can you benefit from bitcois without selling them to a new investor?