It's important to note that price, the last traded price, and value are all distinct concepts. The last traded price is the only one of these we can measure ahead of time, so we often use it as a metanym for these other concepts. But we don't know the price of something until we make a binding offer on it and it is accepted; that doesn't necessarily mean the next transaction won't be very different, we just have heuristics about how the next price ought to look. But it's the process of negotiating a deal or bidding on an auction that defines the price, not the historical prices. If you go to a grocery store and see that milk costs $5.25, and you remember last week it costs $5.00, you don't say the price is wrong; you understand that this is the offer the store has made you, and that the price has gone up since your last visit.
When the oil futures went negative, it wasn't the case that the value of oil was negative - this was about the structure of the market and the sorts of positions people were caught in when the pandemic hit. We continued to consume oil throughout the pandemic, and so I'd say we continued to value it.
Another example would be, if a rancher has a lot of cow poop, they might pay a farmer to take it away. You could say that the cow poop has a negative price on it. But the farmer is going to make use of it as fertilizer; from the farmer's perspective, this is a commodity that has value.
It's true though that there are a million theories of value and it's impossible to say what something's value is definitively, you can only make a decision about what is value is to you.
But when wash trading is and to impact the perceived value, it does so fraudulently; it supplies information to traders - "there is a lot of interest in this security, and the (last traded) price is rising" - which is a lie. Some people will argue this too is a normal and healthy part of markets, but I don't think they've given enough thought to what sort of equilibrium that game will settle into.