That often happens in a free competitive market. I had it happen once and since kept my ideas to myself until I was ready to buy or not. I harbored no hard feelings then as now as this is first and foremost a business, not a high school where we feel slighted that someone "took" our girlfriend from us.
Bottom line, if it is unattached, it's fair game.
To be honest, I wouldn't buy SpaceShipThree.com let alone the other extensions, so to me at least, less than reg fee. Someone saw value in it, though, so good luck to you and the other person whoever he or she may be.