Currency has value because people store it. That is, it has value because people do not immediately liquidate units of A to get units of X. If no one wants to hold A, A has no value. People hold currency/gold/shells because they anticipate that it will have value in the future. Collectively, this gives something a global value. So, I reject the notion that value comes solely from ease of convertibility. Gold is a perfect example.
I disagree. Something has value because people will buy it. A currency has value because people will buy it in exchange for goods and services, or other currencies. People are willing to buy gold, thats is the reason it has value.
Yes, value comes from people. And people are willing to buy Bitcoin, gold, and many other currencies/assets. Buying things with gold via point of sale is extremely rare, however it still has value. I believe you are confusing point of sale transactions (basically, barter transactions) with currency transactions. Barter transactions are not necessarily a good indicator of the value of an asset (as is the case with gold, barrels of oil, porkbellies, etc).