Only if you take an extremely tautalogical definition of "worth" of limited usefulness. If you strengthen it even a little bit further, towards a definition that takes into account a degree of repeatability, then a single transaction is not sufficient to establish worth.

That also accords better with general English usage, where what something is worth and what a particular person paid are not considered generally equivalent. There exist phenomena such as "overpaying" and "getting a great deal", where payment in a particular instance diverges (in either direction) from this more robust notion of "worth". Indeed, if that were not the case, arbitrage would be impossible.

It was worth $4000 TO THAT ONE PERSON. But that's hardly a robust definition of the item's worth or value across an interesting population.

You are assuming that people are rational. Yes, a rational person purchasing an item is evidence that the item is worth the price that was paid for it. No, a person purchasing an item is not evidence that the item is worth the price that was paid for it.