The same issues apply for thinly traded "penny stocks", and you'll see similar caution about believing market cap numbers for those companies. For blue-chip stocks the market is deep enough that you can more-or-less trust the market cap number. But yes, the only "true" valuation for anything is what someone will pay for it, the only way you can get a real answer to "what's the value of all stellars outstanding" is to collect a bid and an ask on that.