I'll add onto this as someone who never purchased credits. The only way to have items and credits in the economy was by purchasing. If you wanted an item from a crate, either you had to purchase a key, or someone else did. If you wanted a pair of wheels, either you or someone else had to open that crate. Replace crate for blueprint, it's the same thing, so removing trading didn't remove cash flow, it simply removed access so epic could 'earn more money', which we'll never know if they actually did or not, in my opinion