The clarification from staff spells out that Crown Store items are considered in-game items, and trading them falls under the same header of trading in-game things. There’s also some protections in place for players who feel that they’ve been victims of a scam, so the staff suggests that you make sure any discussing happens within logged in-game channels. Still, it’s good to know that the items off of the Crown Store can be traded between players for other in-game gewgaws.
Source: Official Site; thanks to Tom for the tip!