> The "market cap" of Bitcoin is a meaningless concept - trading is thin enough there's no way you could ever realise the supposed value of a large holding. Single trades can send the price up or down $30.

I just checked the charts on Kraken which according to https://coinmarketcap.com/currencies/bitcoin/#markets has about 5.4% of the trading volume: https://www.kraken.com/charts. If you want to reduce the price by 30 Euros you'd have to sell coins worth a total of 775k Euros at once (using EUR instead of USD for the comparison, as Kraken's USD volume is only a small fraction of the EUR volume).

Also, according to https://coinmarketcap.com/ the total BTC trading volume over the last 24h was $187M. However, this number only includes exchanges that charge fees (as the volume in exchanges that don't charge fees could be artificially inflated), and thereby excludes almost all of the large Chinese exchanges. The top 3 Chinese exchanges all report volumes of over 3 billion USD during the past 24 hours.

So yes - single trades could definitely bring the price up or down by $30. But in practice most of the traders probably won't be affected by this.