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?
This is always a topic that worth debate
In fact, every investment is a ponzi scheme, since you always have to find the next one who accept your payment, if you can not, it basically become worthless. Many such schemes (like pension fund and social security fund) do not collapse, simply because government can force people to buy into such scheme forever
Most of the fiat money is backed by government (once the government is replaced by another government, their fiat money will worth zero over night), but have you seen anyone go to government to exchange anything? None. So, the backing is just something that gives people confidence. It is better to say fiat money is backed by confidence, that it will be accepted by other people in the country that the government rules
And where does the confidence of bitcoin come from? Suppose that its technology is without flaw, then similarly, the confidence comes from the acceptance by other people in the world. Although this time, it is not by the people ruled by a certain government, but by the people who have confidence in its value
The core question is, what gives peopel more confidence? A government or knowledge and thinking. So far I could say it is still the government that gives people more confidence, but is that a good thing?