Now if Classic had other things in it besides just hats and mounts, Like leveling, Unique weapons, Like an MMORPG or atleast how many work with the "Hack & Slash and Grind' It would work, And has been proven to work in so many games that are extremely popular. But since Classic only has pretty much cosmetics, And nothing truly valuable there wouldn't be much reason to add trading since it would kinda destroy the "Rarity" of an item when they will probably make a similar thing like it in the future.
But on the contrary, For Classic, It could prove beneficial in Classic's stance money wise since I do easily see people willing to buy 20 dollars worth of gralats if not more just to get a specific mount or hat they been wanting for years from someone willing to give theirs away. But I feel with how buggy and laggy classic is (Let's not bring up about how there was a furniture crisis epidemic where there was literally furniture duplicating more and more every day and forever ruined our refund system as punishment, Just imagine this with hat trading), You'd probably get where I am going with this
TL: DR: Prob not gonna happen because Classic is screwed with lag despite Classic probably wants to.