They can tell you a chip is or isn't worth an amount BECAUSE it was bought or sold at such and such value.

How else does one establish what a chip is worth except by what someone is willing to pay for it?

I personally would much rather have a nice old "Faro" chip from the Mint than that stupid "Pink Taco" chip from the Hard Rock or the "Porno Queens" chips, but somebody is willing to shell out the bucks for them, so that must be what they are worth.

Or, did I misunderstand your statement?