[digz]

Not totally liquid. That's a lot of bitcoints being sold. The buyer could not go ahead and sell all of those coins immediately after getting them without severely impacting the price. The buyer would probably need a couple of weeks to unload those BTC without moving prices much. That creates risk as there's a lot of volatility in BTC. As a result, the range of possibilities on what you would eventually sell those BTC for is wide. If you could (and maybe you can... I'm not super aware of all the BTC instruments these days) buy/sell BTC derivatives (options/futures) you would be able to transfer that risk to someone else, but it would be at a cost. This cost would roughly translate to how much you would be willing to pay to be able to sell a bitcoin at a future date at today's market price.

As a result, a buyer would be willing to pay market price - transaction costs - risk transfer cost.

Pricing the risk is of course tough as future volatility is especially tough to predict for BTC. But this is the fundamental rubric for how one would view the transaction.