An investor might determine what they would be willing to pay for an asset (i.e. a fair valuation) and compare it to the market price. If it's selling for less than what they believe to be fair market value then it's a buy. The same methodology can be applied to sell decisions.
If I buy a house to let, I can create a model to project future cash flows derived from rent, costs, taxes etc. I can then discount the cash flows (DCF) with my target rate of return to get a valuation. Yes there are variables that are difficult to take into account i.e. likelihood of government default. It's not possible to apply a cash flow valuation to BTC.
Yes there is a market which determines the price of something. But that does not necessarily result in a reasonable price.
The market determines the price of anything based on supply and demand (which includes a risk calculation).