They did this with NM drops in 11.

Remember the droprate?

yeah, that's because over many years, if the droprate is good, the price goes down to worthless.

They later then changed the items to rare/ex to prevent the bottoming out of prices on non-rare/ex items.

If you want 2% droprates on NM's and bosses, by all means, let everything be tradeable. Otherwise, think before you type.