this is a good definition for someone like me. if I had to liquidate all my assets today, I could easily figure out how much they are worth just by looking at existing bids.
while no less "true", this definition isn't very helpful at scale. warren buffet can't just sell all his berkshire stock by filling orders. depending on the circumstances of the sale, it could either be worth a lot more than n * last price or a lot less.
Same as stock grants: if you're paid in stock, you pay income taxes when that stock is granted, at the market value of that stock.
You aren't taxed on the FMV increase of that stock until you sell it (at which point you're taxed on capital gains).
In the 90s there were stories of a fan who caught a milestone home-run baseball and because of its sentimental value didn't want to sell it, but had to do so in order to pay the tax bill.
In the extreme case, if I have a child, does that constitute income equal to the price a human-trafficker would be willing to pay for him or her?
I don’t think that’s quite true - I would say a better definition would be that a company is worth what someone will pay for it, regardless of if you actually sell it or not.
Stocks, piles of gold and cash are just different types of asset all of which have value.
And you have to really tax all of that, otherwise the wealthy will just avoid taxes by being paid in untaxable gold bricks and trade those for purchases rather than dollars.