Where can I put 'Tradablity' tag in item def?

[WBPark9]

I can't find location

[Ark]

I'm sure you have figured it out by now, but spell it correctly as <tradeability>.
Have a look at Data/Core/Defs/ThingDefs_Items/Items_Exotic.xml for example.
Valid values are "None", "Buyable" and "Sellable". By default items are both buyable and sellable.