If I want to murder people does murdering people have value?

The net value to society is likely negative since the impact on the target is large, and a society that allowed such things may see time horizons constrict.

You raise some good points, though. I personally am probably coming with an unspoken understanding that 'given that something fits into a legal and moral structure, and we accept a certain definition of using something by choice as value' then paying more for that thing implies more value. Facebook is probably a good example of something in the grey area, connecting people has value, but generating demand is questionable, spreading misinformation is likely a large net negative.

Monetary value reflects demand, but demand does not always reflect utility or goodness. There is a demand for atrocity.

The construction “NFTs are highly priced, thus they are desired, thus they are good” is full of holes. If I have a phobia of spiders, and a corresponding desire to eradicate them, does it follow that eradicating spiders is good? If I got my wish, I and the rest of the world would be worse off for it, but my desire exists none the less.

It can definitely have value. That's why bounties exist (or imagine a market for someone to assassinate Hitler).

I think there's a conflation of value and value in a particular moral dimension here.

If the original comment was saying “crypto has value because people are willing to pay for it” that is tautological. There is a conflation, but it’s a motte and Bailey.