Know how to buy but not how to sell.

$PayPal (PYPL.US)$ Actually, there are opportunities in trading options. Unfortunately, I know how to buy but not how to sell. Last week, I didn't sell at 5k, it dropped completely. Luckily, I sold half at 3k yesterday. Got hammered today, hoping for a miracle tomorrow. Why do I always end up holding until doomsday???