sounds like quite a sweet deal, 13inch fish, for a 30G tank with basically everything... but at the same time, i guess it depends on how much you like the fish, or if you need a 30G tank more...
Good for who you our the fish. Do you have the adequate tank size to house a full grown clown knife. If not do you plan on a upgrade.
Does the person you are trading the 30g for have a adequate tank for the clown knife. If so then make the trade.