This can be solved through requiring collateral, and margin calls, which leads us to the real problem: cryptocurrencies are so volatile that too much collateral would be needed, and even then there'd be a rather large risk of default in case the market moves by too much.
In future markets every day your position is marked and your balance updated with your profit or loss.
You have margin requirement and when you no longer have enough in your margin account your position is liquidated so you don't wind up losing more money than you had put in.
