To contract for the sale of securities or commodities one expects to own at a later date and at more advantageous terms.

卖空：订立合同出售自己在以后的某个时间才会拥有的证券或商品以便获得较有利的条款

To underestimate the true value or worth of

低估，轻视：低估（某人或物的）真正价值，轻视

Don't sell your colleague short; she's a smart lawyer.

别低估你的同事；她可是个精明的律师