Quote from: JorgeStolfi on June 12, 2014, 07:07:49 AM

By the way, I suppose that the prices off-market for "bulk" trades are not much different from the open market price. If the off-market price was much lower, there would be people buying bitcoins in bulk off-market to sell them little by little on the open market.
Conversely, if the off-market price was much higher. Is this correct?

Motivated buyers/sellers will pay a high premium for liquidity. You can't buy 100k BTC from the market and expect a good outcome. Anyone who is selling 100k BTC is probably NOT looking for a quick flip. They are looking to establish a large long position without a lot of slippage. Coins that do not float do not affect the price. The relationship between the off-market price and the market depends on which party is motivated, and which party is opportunistic. The impact on market price thereafter depends on whether the seller replenishes, or the buyer reduces. Suppose that I want 50k BTC, but I am patient. I find 100k of liquidity at a discount. If I am capitalized, I will buy 100k and leak 50k into the market, as slowly as my cash flow requirements allow. Similarly if I wish to reduce by 50k but I find 100k of liquidity at a premium, then I will sell 100k and gradually buy back 50k from the market, to exploit the differential. Both of these overshoot scenarios are more likely than the arbitrage scenario, because in the arbitrage scenario, the originator of the liquidity would be better served by using the market, patiently, while the liquidity consumer is already known to be desiring a position -- no one is going to the expense of locating good off-market liquidity unless they want the position.

TL;DR: Liquidity is valuable. You have to search it out. That cost is incurred because of a desired end. It is not free. (Infer: The seeker will not act contrary to the implied goal.)

P.S. I used your terminology of "off-market" which is, pedantically, incorrect terminology. More colloquial and correct terms would be "off-exchange" or "OTC" in place of "off-market".