And the inverse; if you buy at $200 and the stock is at $400 two days later there's a risk that the person you were buying it from has disappeared (maybe they were a short seller and went bust; maybe they got seller's remorse). So RH has to reserve not just the $200 you paid for it, but also another $200 to buy the share on the open market when the counterparty fails to deliver. That's why collateral requirements are based on volatility, not momentum.