If it is within the glass, not extending to the surface, it's likely a heat check. These occur when the handle is attached to the rest when they are at slightly different temperatures. They may or may not grow (some say they don't), and often they don't seem to affect the structural stability of a piece. In my experience they may devalue a piece slightly but not drastically as long as it's clearly a heat check and entirely internal, but it may depend on what you're selling.