Applying a Sigil to an item causes that item to become account bound.
http://wiki.guildwars2.com/wiki/Sigil
You clicked through something when you applied the sigil. That little something was a warning. It’s weird that you could trade it with a friend. If you CAN trade it with a friend (who does not share your account), than it should be sellable. Try putting the skeggox in an another inventory bag (ie. not an “invisible” inventory bag, as you cannot list anything on the TP that is contained in such a bag).